This can happen if the master boot record is corrupt. For example if you install the drive as master, and then change it to Slave. Don't get me wrong, many other things can cause it too. First check to make sure it is the master, perferably the only drive on the chain. Perhaps even use a new IDE cable. If that doesn't help, get a boot floppy, Win98 or ME, or even if you have the strange-o XP boot disks. At the a: prompt type
fdisk /mbr
Hope it helps. If it doesn't more details are required. What happened between point A when the computer worked fine, and point B, where it won't boot?