1. what is the brand of the pen drive?
given the brand, we can check its website and download the software for correctly formatting your pen drive so it works correctly again
2. if you cannot determine the brand of the pen drive, try using the "HP USB format tool
the download is here HP USB Disk Storage Format Tool - Free software downloads and software reviews - CNET Download.com
the HP USB Format tool is created by HP (Hewlett Packard), it allows you to format and hopefully fix pen drives.
i hope this helps