A hard drive may be inaccessable due to physical/mechanical failure, which is different from logical errors on the disk. This article relates to a drive that is physically or mechanically failing. You have a last chance to recover data before the drive fails completely, making the data inaccessible without actually opening the drive.
Stuck Read Heads
Sometimes the heads stick to the platter causing the drive to lock up. If this is the case hitting the hard drive can knock the heads free. This is a temporary fix and should only be used to recover the data. Put your ear to the drive while trying to boot or simply access data. If you can hear servo read head movement then do not attempt this. If the drive is spinning you will hear it turning. Some drives are louder than others.