If your USB Flash dirve or Pen Drive or Card is not recoginzed by computer then you can fix it easily with two steps...
1. Write 'diskmgmt.msc' in the searh box without any quotes and press Enter.
Run ----> diskmgmt.msc ---> Enter
2. Right click on the USB Flash drive letter that appears on the diskmanagement Pop up window and Press change drive letter.
Diskmanagement ----> Right Click on USB drive letter ----> Change Drive Letter
3. Change the drive letter and Press ok and remove your flash drive and insert it again and now it will appear and your USB Flash Drive ( Many People call it as Pen Drive) will work again.
Remove Flash Drive ----> Insert It Again ----> It will now work.