OK, sounds like the disk got corrupted, you'll need to format it to get it usable again. You'll lose your data on it though.
Put the card in the camera, and look in your manual on the steps to format it for use. If the camera does not see the card at all, format it in the PC (right-click on it's drive letter, and select Format. Use FAT32 file system), then try it in the camera again.