I had same problem before and rebooting solved it for me. Try the following hopefully it will work otherwise you are F@$ked.
In Window XP, go to start, All Program, Accessories and then window explorer. See if you can open your drive this way. It may sound silly, but last week a colleague had this problem and we opened the files this way. Good luck.
If it did not work try, Start, All Program, Accessories, then c:\command prompt. You will be in DOS prompt. Then at C:\******> do the following and you need to know the drive name e.g D, E, or J.
If your drive is E then type the following.
C:\******> dir E:
then hit the ENTR and see if it list all the files on your USB key. If this works, then maybe you will be able to retrieve the data.