I agree... the first thing you should try is a fresh battery. A four year old battery might be on it's last legs (probably depends upon how many times it's been recharged.... and how deeply it was discharged in between charges).
If possible, it would be good to also try a different charger. Sometimes they go bad.
Does your camera have a separate replaceable date/time battery? If so, it might be a good idea to replace that, too. Those types of batteries usually have roughly a five year lifespan.
Wouldn't hurt to update the firmware. If' it's never been done, very likely there is a newer version. Even if it's not an update, it might be a good idea to reinstall it.
HOWEVER, before you attempt to install a firmware update, you must have a reliable, fully charged battery. You don't want to have a camera shut down halfway through a firmware installation. That's almost guaranteed to "brick" the camera. So sort out the battery first, to be certain you don't have problems there.
The reason for replacing the firmware is it might have become corrupted. It's essentially an operating system for the camera, which is essentially a computer... and "stuff" happens, as we all know, to computer operating systems! Sometimes a reinstall or update will correct problems in the software.
Canon's and ebiggs instructions above about "rebooting" the camera are a good idea too. In fact that's the first thing I'd try, since you don't need anything to do it. The reason you "reboot" is to clear any lost data strings from the memory, that might be interfering with the camera's operation. Again, it's a lot like a computer.
A little more about rebooting the camera... If your camera has a date/time battery, in addition to the main, rechargeable... remove both of them. Rather than letting the camera sit, another method is to turn the camera on without any batteries, and press the shutter release button once. It won't fire, of course. But this should drain any remaining charge from the circuit immediately and is faster than waiting for the residual charge to drain off slowly.
Once you have done this (or let the camera sit for 20 minutes or several hours without the battery), go into the menu and see if the date/time needs to be reset. If so, you got a proper reboot.... If not, try again.
In addition to trying a different memory card, always formatted in the camera, also inspect the slot the card fits into carefully. Look at it with a flash light, to see if anything looks damaged, shorted out or if there is any obstruction. Probably not something you can fix yourself, if there is a problem.
I agree about cleaning the lens to camera contacts. I use a few drops of isopropyl alcohol on a clean, lint free cloth. That's to remove any oils (finger oils, or lubricating oils from inside the lens or camera). Just keep off the mirror in the camera and the lens optics.
You also might use the same to clean the battery contacts both on the battery and on the charger (unplug it) and inside the camera. Wipe down the date/time battery, too.
None of the above is certain to fix the problem, but all are fairly easy and free or relatively inexpensive to try.
If none of the above works, then send the camera in to Canon and have it checked.
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
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