Without more information about the hard drive and motherboard the best advice i can offer is to check all connections make sure everything is firmly in place. you might also like to check for bent pins and such as well this ofcourse will depend on if your using ATA or IDE connections.

It may seem obvious but check in your bios to ensure that your Hardrive is
a) recognized under drive configuration (or something of the same ilk)
and
b) listed under boot sequence

If that doesn't work, provide some more information about your computer (everything you know is always a good start).

Kristin

If you have nForce4 chipset and SATA II drive, that is normal. You'll need to switch the jumper on you HD to SATA I. (NVIDIA's fault)