|Look for the flash drive in Windows Disk Management and post all the information shown for that drive. |
I suspect the partition is corrupt. Is the partition type RAW? If so, you can probably fix the partition without loss of the files. There are two utilities that you can try.
The first is Easeus partition recovery. See first link below.
The second one is called Testdisk. Be sure to read the documentation for both before use. Second link below is for Testdisk.
You may also be able to recover the files using a live version of Linux.