|Check in cmos/bios setup to make sure the IDE controllers are enabled.|
If they're OK, open the case and see how they're connected. Are the cdrom and hard drive on the same cable? If so, and if they're jumpered wrong that could cause them not to be seen.
Are they connected to the 4-wire power connector?
Post back the model number of the hard drive. If the drive was replaced with a larger capacity one it's possible the bios won't be able to see it.
You're not really green until you're soylent green.