It can be several things. :(
A dead hard disk, changes made in the BIOS to the hard disk parameters or Primary IDE controller, corrupted Master Boot Record and/or boot files, inactive Primary Partition, bad or damaged flat cable, bent pins on the hard disk or IDE-controller (now a days usaully onboard of the MotherBoard), maybe a dead IDE-controller.
If you're able to boot with a bootdisk, and from there access the hard disk without problems, you could start with a simple:
fdisk /status[Enter]key, check if the Primary Partition is set active. (A)
sys c:[Enter]key, reboot without bootdisk.
Note: the bootdisk, must be for the same version of the installed OS.
If sys c: doesn't solve the problem try:
fdisk /mbr[Enter]key followed by
sys c:[Enter]key, reboot without bootdisk.
Hope this helps.
Greetz from The Netherlands,
The Count