The first thing I would do is try another card. Borrow one from a friend, or find a cheap one to try. Whatever works for you, but this will help determine what the problem is. If the other card works, then your sd-card probably stopped working. You could try formatting it in a PC but you will want to make sure to back up all the sd card contents to the computer before formatting, otherwise all of it's contents will be deleted.
If a second sd card doesn't work either, it is likely a hardware issue with the phone and will probably need to be repaired or replaced by your carrier or the manufacturer depending on what kind of warranty the device is under.
