My guess is your PC wasn’t shut down cleanly during a write operation (a crash of some sort) or the drive wasn’t ejected safely.
You should be able to fix it by opening a CMD window and issuing the command
chkdsk /f (drive letter):
where (drive letter) is whichever letter windows assigns the drive when it’s plugged in.
Such as,
chkdsk /f e:
or whatever.