Have you gone into your control panel, and clicked on Hardware and Sound? From there it should list the options your computer has for sound. Is anything checked there? Can you select the default option given?
Also, have you rebooted your computer since you lost your sound? There are times when my sound/speaker icon on my desktop just disappears. I can still manually go in and control the volume, but its less a pain in the rear to just reboot and it loads up for me at the time.