hard disk

Your OS (operating system) discs should have "cabinet" files with generic device controller software--referred to as drivers. If you are using the Windows operating system, go to "Start" and look on the right hand side for "Control Panel." On the Control Panel page, look for "Add Hardware" at the top of the page. When you click on that, you will be asked what the hardware is, and you will also be provided the option for Windows to look for it. If Windows finds it, then you will be prompted to provide a disc with the device control software--the drivers. You will also be given the option for Windows to look for the drivers. That is often a better option, as Windows is cranky, and likes its own generic drivers best. If you go online before you start the procedure, Windows will also look for drivers on the internet, if you tell it to do so.

If Windows does not find the hard drive when you tell to look for new hardware, then there is a high probability that it is not properly installed, or not connected to the mother board. If you don't know what you're doing when you open up the case of the PC, then you would want to seek professional help at the point (for the PC, not for you).

You don't offer much info; what you're asking is sorta like saying "My car won't start - help me"

A partial list of possibilities -

drive failure

poor connections

improper jumper settings

corrupt BIOS or other corrupt software

1999 and earlier BIOS generally will not recognize drive larger than 32MB