Plese open the device manager , and see if you have any unrecognized usbb storage device there. If yes, uninstall it , remove your pen drive , and then reinsert it.
You can also see in the Disk Managment . Is there an unassigned ( drive letter ) device there . If yes just assign it a drive letter.
Floppy Drive to USB
Floppy Disk to USB
Floppy Disc to USB