This is an entire area of specilization. You might want to do a gurgle search for help.
Rescue of USB flash drive files is not for the weak of heart.
There are so many causes. Failure could result from something as simple as pulling out a flash drive too early while it is still recording or downloading files, or it could be damage to the hardware around the USB plug.
Rescue is rare, so if the data is critical, get the help of an expert... Most repairs require the destruction of existing data on the flash drive. So do not do anything more until you do some research. Some brands, such as SanDisk and no-name units are particularly susceptible.
While other brands will rescue the data if you mail them the drive.
Some companies specialize in rescue of the data, but they usually remove the memory element, which means they destroy the drive in order to save the data.