Try cleaning the card with some alcohol and reinsert it. Make sure it actually works on some other device. For example, can you read it on a PC using card reader or in a different phone? Is the card formatted to FAT32? Reseat the card several times. If nothing else helps, you will have to hard-reset your phone. Make a backup of all your important data before you do that. But in most cases, this helps:
a) Touch the arrow icon on your start screen.
b) Go to "Settings".
c) Choose "About Phone".
d) Choose "Reset your Phone".
e) Choose "Yes".