Check in the BIOS to see if the Hard Disk type is set to Auto. If it isn't, set it to Auto.
If it's set to Auto, and the drive is still not detected, check the IDE (flat grey ribbon) cable to it. If there is a CD-ROM or another drive on the same cable, remove the cable from the other drive and recheck.
Also remove the cables and refit (don't take them out of the case - simply take off each end, and push back on). Make sure you try a different power plug too. If you have another IDE cable, try that too.
If all the above fails, then you probably have a faulty Hard Drive.