May I offer you a warm welcome to the CameraLabs forum.
That's bad luck and dreadful timing. I've not seen this and I'm no expert but it sounds as though the card reader in the camera may be at fault, possibly with a bent pin. If that's so I see no option but to send the camera in for repair as, and I proved this to myself pretty disastrously a few years back with a card reader on my printer, trying to straighten a bent pin is a very risky procedure.
But maybe SD card readers are more forgiving than CF readers in that regard?
The only easy thing I can think of for you to try is to remove the camera's battery for a few seconds just in case this is a software glitch in the camera.
Failing any better suggestions on how to fix the problem from other members I see no option but to have the camera repaired. Can you borrow another Canon DSLR from a friend for the graduation? Another possibility would be to hire a camera but that's an expensive way to go unfortunately. Fingers crossed that you find a solution in time for the big day.