When did the problem start? Just suddenly or after you installed / changed aynthing?
Format the hard drive... But...
This will blow away all files on your drive.
That's a bit drastic to start with! There's a LOT of things you could try first. To start with:
Try going into the BIOS, check whether the drive is listed there. If so check its SMART status. If the drive isn't listed at all then have a look inside the case, check the cables are properly seated and plugged in firmly. It might also be worth trying it in a different socket, with a different cable or in another computer altogether if you have access to one. If none of these fails to recognise it it's almost certainly dead.
If it's listed and the SMART status is OK, grab something like knoppix and boot up from that, see if it recognises the drive and if it can access it. If so, then you know it's a windows problem and can go from there, again if knoppix can't recognise it then try it in another PC and check for results there.