When the system starts up, you should see something like:
Primary Master: .... SAMSUNG HDA219890234 ATA-133
Primary Slave: ..... Not Detected
Secondary Master: .... LITEON DVDRW
Secondary Slave: .... Not Detected
Are you missing the one that says "Primary Master"? You may need to go into your BIOS to see it clearly (usually press <DEL> repeatedly while system is booting up). If it doesn't show up, it means that the disk isn't detected.
Open it up, and see if everything is plugged in correctly to the hard drive. There should be a power cable and an IDE cable plugged in. Reseat both of them (unplug, plug back in). Power it up again, and have a listen... Does the hard drive sound like it's spinning up? If so, you may have other problems (bad disk, head crash, etc).
If the drive is detected above, it may be that you've lost your MBR or partitions, or your disk was formatted.