Open your Control Panel and go to Device Manager. You might see an unknown device or one that's currently not working there. If yes, that's probably the missing audio device. You can rightclick it and allow Windows to search for a driver to re-install it.
If you don't see a problem device, expand the Sound, Video and Game Controllers tree. Is there a problem device listed there?
If you can't find it, try Scan for Hardware Changes from the Action menu. Please report back with whether you're successful.