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Try what Kelmar suggested. Hopefully it's just a dirty contact or something equally benign. If that doesn't work, try reading the sd card in a PC card reader. If you can get it to read the card, copy ALL of its contents to a folder, including hidden files.
Then you can put it back in the phone and press menus>settings>storage (it might be slightly different depending on which phone you have) and reformat the card. Then put it back in the PC and copy all the files back to it. Make sure when you remove the card from the PC you cleanly unmount it. (open My Computer, right click on the drive letter that represents the card reader and choose eject.)
When you put it back in the phone it should work fine.
If none of that works, I fear the card is bad.
Good luck.