First make sure that your BIOS can see them. When you're booting up you will see your computer's BIOS listing drives that it has found -- you might have to put it in verbose mode before it will show you (but first yell at it for showing you a logo of your computer manufacturer instead of providing useful information). You can also see your drives in your BIOS itself (the access key depends on your BIOS version).
If your BIOS can see the drives then you know that it is indeed Windows that is having some difficulty seeing your drives. This may be caused by bad/corrupt drivers. Go to Device Manager to reinstall the drivers --
How to Troubleshoot Hardware Problems with Device Manager.
If your BIOS cannot see the drives your drives may not be inserted properly. You'll have to open up your box to see if all your IDE (I'm assuming your disk drives are hooked up via IDE) cables are tightly inserted to the back of the drives and to the motherboard itself (don't push too hard on the IDE cables -- you might break your mobo). Your IDE cables might have failed too (but this is probably not the case.)
If neither of these options work I'd assume that the drives have failed and replace them with other ones. If you live in a city (or near one) you can usually find shitboxes (old, but usually still functional computers) to take parts from via dumpster diving.