|Don't be too quick to condemn things. Here's some real life troubles I've found over the years, when hdd's and CDROM's dissapeared.|
Reseat the cables--that is disconnect and reconnect the data (IDE) ribbon cables.
Reseat the power connectors. If you are "handy" with small tools, and needle nose pliers, gently squeese the contacts for better connection.
If not, find an unused power connector lying in the case, and use it instead of the one previous.
Don't be afraid to try a different ribbon cable. For quick troubleshooting, you can pull the one from the CDROM, and hook it to the hard drive, assuming both are IDE.
IDE cables are cheap--don't be afraid to replace the cable.
Reset the bios. There should be a procedure for your motherboard, and you should have the manual. IF not, you usually can look the manual for the motherboard up on the 'net.
If you can't do that, then unplug the power from the computer, and pull out the "bios" battery, and leave it for 1/2 hr or so.
Try to determine, when you power up the machine, if you can hear the hard drive spin up.
If possible, you can even try the hdd in a different machine.
If all else fails, it actually would not hurt you to have a second hard drive for storage--everyone can use more space
So consider buying a second drive, and see if the bios will detect THAT one.
(I just don't know what I'd do without all my "spare" thrift store machines that I'm able to use for testing and troubleshooting.)