can you give us some more information? I take it if you plug you camera into your PC you can't access the card?
Have you tried taking the card out and reinserting it (i'm sure you have)?
Do the contacts on the card look clean (no lint or fluff).
Assuming the card still works you may need to format it as the filesystem may be damaged. If you have data on there that you need it is likely still intact, you should try to recover it first with some "undelete" or data recovery tool.