First remove battery
and card and look in your flash
card slot for any bent pins. If you see some, don't fret. Use a small/skinny jeweler's flat screwdriver to bend the pin back as close as possible. Take your time and use as much patience as you can muster (take a breather in between if needed).
It doesn't have to be perfect, just as close as you can get it. When it's there, take your flash card
and "SLOWLY" insert it. The card itself should fine-align the pin. Remove the card, and verify that the pin is still straightened. If so, reinsert the card and try it out.
If there are no bent pins, then the internal BIOS battery has likely run down and is causing this error. Don't attempt to fix this yourself. Canon
will fix this for you for free, with free shipping, regardless of your camera's warranty status. See and print out the following advisory:http://www.usa.canon.com/consumer/controller?act=PgComSmModDisplayAct&fcategoryid=223&modelid=8777&keycode=2112&id=27465
Also, see the following link for tips on contacting Canon regarding camera advisories. This particular post concerns a CCD advisory, but the tips are the same:http://camerarepair.blogspot.com/2007/11/canon-digital-cameras-showing-black.html