MCs are easily corrupted - even the official ones.
Vice City used to do this all the time to me - original and backup. If you have a chip, download and burn QA Tool - a free app (available from Alucard.cc) which will allow you to format and then unformat your card. This returns it to its out-of-the-packet state. (you'll have to go to browser and format the card afterwards).
Your card will then be back to normal, but you'll lose your saves
Good luck!
