Occasionally, a PC's CD or DVD-ROM drive will stop responding for no known good reason. Sometimes the problem is hardware--a CD-ROM drive, being a mechanical component, can fail--but as often as not, it seems, the problem is software rather than hardware.*
Troubleshooting an IDE CD-ROM drive that quit working is pretty much the same under Windows 95, Windows 95B, Windows 95OSR2.1, Windows 98, Windows 98SE, and Windows Me, whether you have a Compaq, Dell, eMachine, eMachines, Gateway, HP, IBM, Micron, Packard Bell, Sony, or clone PC.**
In an effort to avoid tech support calls about DOS games, most computer manufacturers load DOS-mode CD-ROM drivers in a file called config.sys. This reduces tech support calls but increases the chances of software failure by about a million percent. Open My Computer, navigate to Drive C, and locate the file config.sys. (It might just say "config".) Rename it to something else.
Don't reboot yet. Often, this will solve the problem in and of itself, but frequently there's another problem on Windows 9x boxes.
Sometimes Windows has trouble deciding whether to use the driver specified in config.sys or its own built in driver, so it'll bluescreen. Then, the next time you boot, it adds a key to the registry that disables the CD-ROM drive entirely and makes the rest of the computer run about as fast as a Studebaker.
Nice of it, eh? To check this, click Start, then Run, and type regedit. Double click on HKEY_LOCAL_MACHINE, then navigate to SystemCurrentControlSetServicesVxDIOS. You'll probably see a value named "NoIDE." Right click on it and select Delete. Reboot. Your drive will likely come back to life.
If neither of these things work, you can determine for certain if the problem is hardware or software through a couple of methods. If your manufacturer gave you a restore CD, try booting off it. Hold down the shift key while it tries to boot in order to prevent it from doing anything nasty to your system--you just want to see if it boots up. If it doesn't boot, you've got a hardware problem. Replace the drive. If it does boot, either try the above directions again, or you've got a problem I've never heard of.
If you don't have a system restore CD, you can accomplish basically the same thing with a DOS boot disk. You can get one of those from bootdisk.com. Boot off the disk, pay attention to what drive letter the CD-ROM got (usually D: but it can vary), insert a data CD, and type the command DIR D: (substitute the drive letter that came up if it's something other than D:). If you get an error message, you've got a hardware problem. Replace the drive.
Windows NT, Windows 2000, and Windows XP are immune to the problems I described here. If the drive quits working under one of those operating systems, either your drive lens is dirty or you've got a bad drive. Cleaning kits are hard to find and overpriced. As I write, an Artec 56X IDE CD-ROM drive costs $19 at Newegg.com, so outright replacement doesn't cost much more than cleaning.
* Yes, I realize this is yet another boring troubleshooting entry. Having nothing interesting to say today, I'm writing entries that I know will get me Googlejuice down the road. Analysis of my early stuff and of some of the comments on this site has made me realize there are still some gaps in the Farquhar back-catalog.
** Yes, this is another paragraph written with the express design of getting hits from Google. If you can think of any search term I might have possibly left out, well, that's what the comments are for.