Put it in your PC and format it FAT32. I would use Disk Administrator and delete, recreate the partition and then format it. If its an old SD card, I would run CHKDSK /R before using it.
It does sound like it got corrupt; you can try a recovery program (PC) to recover whatever was on it--but that isn't necessary because you have a backup, right? Riiiight? RIIIIIIIGHT!?