You should not put files on a pen drive which are not backed up somewhere else, as pen drives are just not reliable enough. Your files are lost & you'll have to buy a new pen drive (but don't use it for storing valuable files you haven't backed up).
As phil22 said, flash drives are not very reliable. Regardless, a person should always store their important data on multiple forms of media. You can try to use a data recovery program to reconstruct it but flash drives are more difficult to recover data from in the event of hardware failure. Your final option is to take it some place such as Kroll OnTrack. They specialize in data recovery, but they are expensive. Figure a starting price would be at least $300, and you could expect to pay as much as $1,000 for something such as a flash drive.
Don’t panic. You could recover your data back as long as you take the right steps.
1. Do not save new file on the same drive in case of data loss.
2. Download a pen drive recovery freeware to regain your data. This application was helped a lot as the external hard drive of my bother became inaccessible. At that time, I was there. This freeware surprisingly retrieved all his files back. You could give it a chance: http://freeware-fix.blogspot.com/2013/03/external-drive...
3. Save your recovered data on a different drive in case of recovery failure.
Note: You should always remember to back up your important data in the future.