Mixer and Sound Devices

I have a NT pc that will not let you adjust any volume settings. If I try to open the volume settings window it gives me an error that either that the pc has no mixer devices or they have been disabled. I went into me devices window and there it told me that the pc has no sound or mixer devices installed. How would something like that happen? and how do I get those devices back? Or how do I reinstall the devices? I tried looking on microsofts web site, but they didn’t have anything to helpme.