Go to Device Manager. Right click on My Computer go to Properties, Hardware, Device Manager. Check under Sound, video, and game controllers to see if you can find anything that resembles a sound controller. If you don't, check under Other devices. If it appears there then it's been uninstalled and Windows hasn't been able to find the correct drivers. I'm guessing you have some sort of Sound Blaster (everyone does), got to the Sound Blaster website and download the lastest drivers.

Or, if you know have a built-in sound card on your motherboard: nForce, nForce 2, nForce 3, etc.; then maybe it's been disabled through the BIOS.

Check Device Manager first, though, and see what you find.