Don't be surprised if this thread gets locked. Im gonna try to help you anyway.
A multitude of reasons exist why you might be having that problem. It is really hard to help someone remotely, because your problem could be with your hardware, or your bios. It could be your harddrive, or it could be your motherboard, or your cables, or your power supply. A big question is: When did it stop working? Did the computer suddenly crash and couldn't boot up again? What is the context in which this problem occured?
A few things to try:
Open the cast and make sure everything is connected and not loose. Make sure the IDE cables are plugged into the harddrive and the motherboard correctly. Make sure the powersupply cable is plugged in correctly.
As for the bios, when your computer boots up, press the delete key to go into the bios. See if your bios can detect your harddrive. Try reseting your bios to its default settings.
A good place to try to find more answers is a forum where people talk about these things. Try this website:
www.anandtech.com and register for the forums there.