First, always suspect the battery on any camera where the shutter is electronically timed. That includes just about every modern camera. Even if the battery tests OK. Even if the customer tells you it's brand new. Try a fresh battery. 

• Make sure it's facing the right direction
• Clean the battery contacts. Even an invisible amount of corrosion can freeze a camera. Rub the battery on a piece of scrap paper, and clean the camera battery compartment by rubbing it gently with a pencil eraser.
• Some cameras need to have the shutter reset after a battery change. Olympus SLRs have a reset position on the shutter ring. Some cameras should have the shutter speed ring moved from A or from the timed settings to "M" or "X", and then back again.
• Not all cameras depend on batteries. Older models with a pebbled glass meter cell generate their own current.

My camera won’t work. When I look through the back and click the shutter, it doesn’t open:

Maybe there’s nothing wrong with it. Nikon 35mm cameras of the EM/FG family don’t open the shutter until the film counter gets to number 1. Obviously, when the back is open the film counter never gets to #1.

Nikon SLR cameras with a "DX" setting on the film speed dial will lock up, in the program mode, when no film is in the camera and you get to #1 on the film counter. True of models 2000, 2020, probably some others.

My camera won’t work. It keeps blinking "E" (or maybe that's a "3")

Many 35mm auto advance cameras  won't work at all when the film is ended. They will rewind the film and refuse to function until somebody takes out the used roll. Yes, this is a good thing - but sometimes the customer thinks the camera is broken. Usually the film counter will show no number or an "E" - sometimes blinking. Looked at backwards, the "E" may be mistaken for a "3." Just take out the film - in a darkroom or black bag if you're not positive that it's been rewound.

Some autofocus cameras have a computer control that occasionally goes "bonkers." The display blinks "E" even when the film is out. It continues even when you take the battery out, because the camera has a small amount of power buffer that holds memory while you're changing a battery. If you take the battery out and ignore the camera - possibly for as long as a day - the display stops blinking and frequently the system resets itself! Granted, it's tough to tell this to a customer…

My camera won’t work. I can't release the shutter 

Manual wind cameras can't be triggered until the film is wound. If the film is at the end - say, picture 21 on a 20 exposure roll - the customer may think she's wound it when she really hasn't. Ways to know: without depressing the rewind clutch, gently turn the take up crank. It won't want to turn at all! How to convince the customer: leaving the lens cap on, depress the rewind clutch. Wind the lever the little bit you need to in order to cock the shutter. Click the shutter. Be sure to remove the film.

Sometimes the film advance lever just wasn't advanced enough. If the lever seems to flop back and forth, try winding it the extra few degrees to cock the shutter.

And with modern cameras, we will always suspect the battery.

My SLR takes 2 or three pictures and then won’t take another for several minutes. When a single-lens-reflex camera has an old lithium battery (CR1/3 or PX-28L) this is not uncommon. The battery will have enough power for two or three shots, then freeze. 20 minutes later it will have recovered enough juice for another couple of pictures. Solution: put in a new battery. We recommend using 2 S-76s instead of the single lithium battery 1/3N, or a silver PX-28 instead of the PX-28L. 

My autofocus camera "locks up."

When autofocus cameras are aimed at a subject that doesn't have adequate details or contrast, they won't focus.

On some models that means they won't take a picture at all.

If a customer brings in a camera and says that it won't focus, show them how to aim it at something on which it will focus, depress the shutter part way, and then recompose the picture.

Don't take in a camera for repair if this is the complaint; it can't be fixed because the problem is not in the camera, but a matter of physics.

My autofocus camera won't focus.

Some of the explanations above apply. Here are two others that apply to autofocus SLR cameras:

If the lens, or combination of lens and multiplier, is not "fast" enough most AF SLRs can't focus. The combination of a 2X teleconverter and an f4.5/5.6 zoom lens has an effective maximum aperture of f9/11 - not adequate for automatic focusing mechanisms. Solution - don't use teleconverters with slow lenses.

If the lens or filter (or back end of the lens) is really dirty, the contrast is degraded too much for the autofocus to work. Solution: clean the lens and/or filter.

"The only other thing I can think of would be with a AF SLR focus problem. They can try to lightly clean the lens and body contacts and make certain the lens is turned when attached until a CLICK is heard. Also try camera in Manual Focus to narrow down to a AF problem area." Ken Blauvelt, Minolta

If the viewfinder looks dark, maybe the lens isn't reopening to its maximum aperture for viewing and focusing. This almost always indicates an expensive repair is needed. Usually the camera can be used in manual focus mode until the customer can afford the time to get it fixed.

My backgrounds always look black (or really dark) when I use the flash. What's wrong with the flash? Answer: probably nothing's wrong with the flash, you're just running into that branch of physics known as "inverse squares."

Light from a point source - such as a flash gun, or a light bulb, or the sun - spreads out as it gets farther from the source. When you're twice as far away, there's only one-fourth as much light (remember, it's spreading out in two dimensions). When you're five times as far away, there's only one-twenty-fifth as much light.

The background is farther away from the flash than the subject. If the background is a lot farther away, it's getting much less light from the flash - so it will be very dark.

You can't solve this problem with a bigger flash or with a faster film. Just take my word for it.

You can solve this problem by reducing the ratio between distance from flash to subject and distance from flash to background. Have the person stand closer to the wall. Or move around so that instead of having a huge open space behind him, the subject has a light wall. Or you can move farther from the subject and zoom in with the zoom lens. Now you'll have the same size picture of the subject (because you've zoomed in) and you'll have less extraneous background (because you've reduced the angle of view) and the background will be relatively lighter (because you're improved the ratio of distance from flash to subject and distance from flash to background.)

You can also solve this problem by bouncing the light from the flash off the ceiling. To be effective, you need a powerful flash - a fast film - and a fairly fast lens. Also, the ceiling must be white and not too high. 

My camera won't work. There's something wrong with the date

With more and more dateback models being bought, the people with cameras that use a separate internal battery should understand that the battery will run the clock feature even after it's too weak to light the "little light bulbs" to expose the numbers for the date. Also ask "is the time set correctly?" when they say the date keeps malfunctioning. When I ask them this I always hear "I don't need that printed on my pictures so I didn't bother to set it". They then go on to explain the "problem" is it keeps changing to the "wrong date" before it's supposed to (or is that changing to the right date too early?). It's very important that a databack is set to the right time. When a camera of Oriental manufacture arrives in the USA, the clock is many hours ahead of the new time zone - so it will change to tomorrows date around noon local time. Another helpful hint from Ken Blauvelt, Minolta

Tell customers in advance when they shouldn't spend money on a repair:

Movie cameras - why bother? They cost too much to use for anyone but the enthusiast, and film is tough to get. 8mm film is out of production. Unless your store stocks the film size used by the camera, why spend money on the repair? 

Polaroid cameras: - most cameras cost more to repair than to replace. Unlike most repair shops, Polaroid will automatically repair cameras without sending an estimate, which may lead to unpleasant surprises. Only a few models make sense - the professional model 180 or 195, the 680SLR.

Instamatic Kodaks and Disk cameras - why bother? Kodak made the last disc camera in 1988, and repairs are at least $28. Disk film was discontinued at the end of 1998. You’re performing more of a service by selling your customer a new APS or 35mm point-and-shoot model.

Petri and Mamiya 35mm cameras - no parts are available.

Good leaf-shutter SLR cameras such as Kowa, Retina Reflex, Contaflex, Topcon 100 - if parts can be found, the cost is still disproportionately high.

Older rangefinder cameras (except Leica, Nikon and Canon models with interchangeable lenses). Few parts are available. Costs are high. Many depend on now-illegal mercury batteries for proper operation.