First off, you should never, ever power off a computer during a defragment as its using the actual heads in the drive. It may appear to have been locked up but depending on how fragmented the drive was, it isn't locked up just working.
Why were you trying to use it during a defrag anyway? When running a defrag, no touchie the computer.
As for fixing it, you can try to repair it, or you may have damage to the drive like Brett suggested.