right click on the volume icon near the clock and open the sound properties. If not showing get to sound system via the control panel. You might find that one of the options has moved completely to one side. If not you may have to reinstall the sound driver that came with either your motherboard or sound card. If it is a separate sound card it may have slightly moved in the pci slot which would mean opening the case and lifting the sound card out and then putting it back in again.