When did it go wrong, and what can you remember doing before it went wrong?
Right-click my computer, click properties. click the hardware tab, and then device manager. may be a little different for you, but you don't state what version of windows you're running.
under the CD-ROM drives section, your drive will be listed (probably with an exclamation or question mark next to it). right-click, and click "scan for hardware changes". if it finds anything, its just a case of following through the options. restart, and see if that helps.
if that doesn't, go back into device manager, and right-click the drive and click uninstall. once its off device manager, right-click your computer's icon in device manager (the very top), and scan for hardware changes again. see if that works..