Looks like the file system of your pen drive became corrupted.
You should try some of the available file recovery applications that are available, several for free. Do a search for
"file recovery" AND download
Including the quotes and respecting the capitalisation shown.
If the file recovery work, copy the recovered files to another location. Reformat the pen drive. If you can't reformat, that pen drive is dead.
If the file recovery failed, those files are LOST. Try reformating the pen drive.
You should NEVER keep your only copy of any file on any pen drive. I just hope that you have the originals somewhere...
Electro, June 2013