My would guess would be that either the usb flash drive either isn't being properly powered (disconnected power pins) or the on board microcontroller may have possibly stopped working..
You can try opening up the drive (WARNING : this will void any warranty) to see if any of the pins on the USB connector are loose or disconnected. If they are you can try to bridge a connection and pull off your files.
If that fails, you may need to use a flash drive recovery service and have them repair the drive or just extract your data off the drive.
For example, http://fixmyflashdrive.com