Whatever the cause, you'll almost certainly have to remove the drive and either send it off to a specialist recovery firm or attach it to another computer and try it yourself. Before you shell out for the professional recovery option you could try using a free utility like Partition Find And Mount.
Partition Find & Mount implements a new method of deleted or lost partition recovery. It works simply by locating and mounting partitions into the system, thus making those lost partitions available to you like any generic disk volume. Partition Find & Mount will also work where any Boot Record (including the Master Boot Record) is missing, damaged or overwritten.
It's almost certainly worth a try if you do have a serious problem with a hard drive and, since it's free, you have nothing to lose but a little time and you could save yourself a lot of money.