Remove and replace your hard drive - if it still isn't found, it'll need to be replaced, as it has failed. Just about any 2.5" notebook drive should work fine as a replacement.

The problem probably is a damaged boot sector on your harddrive, assuming that its not a damaged piece of hardware.  Try booting with any standard bootdisk and then rewriting the boot sector.  If it still isnt working then I suggest you attempt to re plug in the hard drive; otherwise, try reloading the O/S unless you are positively sure that it worked prior to its failure - otherwise it may very well be a hardware issue.