Since this recently happened, two things to try.
1. Use the System Restore and restore the PC back to a date when it worked OK. If that does not work, System Restore has an "undo" option to undo the System Restore.
2. If the System Restore does not work, Uninstall the sound in the Device Manager - RIGHT Click on the entry for the sound device and then LEFT Click "Uninstall". DO NOT check the box to uninstall the drivers
. Restart the PC and when Windows starts it will detect and reinstall the sound.