The underlying NAND flash die has run out of physical write endurance cycles. The hardware is dead and must be replaced.
The firmware acts as the operating system for your USB's controller chip. It manages how data is written to the NAND flash memory and tells your computer what the drive's capacity is. When this firmware "crashes," the drive may still be powered on (indicated by a light or a "ding" from Windows) but will fail to function as a storage device. Essential Tools for the Job Sss6697-b7 Firmware Download
Before you proceed, you confirm your USB drive's controller. Using the wrong tool will not work and may cause further damage. Use the free tool ChipGenius to scan your drive: The underlying NAND flash die has run out