Before trying to fix your USB flash drive, you should make sure that its bad functionality isn't determined by other external parameters. For example, try other computer USB ports or test it on, at least, another computer.
If your USB flash drive still refuses to work, here is what you can do. Right click on My Computer and select Manage. From the right menu, double click Storage, select Computer Management and see if your device appears in the list. If it does, right click on it and hit Change Drive Letters and Paths and change the existing drive letter, hit OK and close the dialogue boxes. If your USB flash drive isn't recognized by Disk Management, it is probably a physical damage and the only thing you can do next is to recover the stored data, by using a data restoration software, such as Pen Drive Data Recovery, http://data-doctor-recovery-pen-drive.software.informer.com/ or Spotmau Data Recovery, http://spotmau-data-recovery-rescue-cd-usb.software.informer.com/