I have had the same problem. Ultimately you'll need to reformat it but before you do that, try running some file recovery software on it and you should be able to rescue some if not most of your files. I found all my pics I took and the only ones I have found problems with is playing a few of the .mov files recoverered from the card.
Try the following programs:
Easy Recovery Professional.
File Rescue Plus.
FileRestore.
Recover4all pro.
Recover.My.Files.
Undelete.
You will probably find that the card is not detected by your pc but if you tell it where the card is I found that works.
Some of the above will require serial no to activate full version, however most will let you scan the card for files and then you will need the serial no to actually allow you to save the recovered files.
I wouldn't get serials from crack or serialz web sites cos that would be wrong! some are only about $19 anyway.
Hope this helps
CoolPaul
December 2007