That sounds like the sound driver has become corrupted. (It's quite common; it's happened to me several times over the years). You'll need to download the driver installation file, double-click on it to run it and then restart your computer. (Alternatively, you could try running System Restore, from Start > Programs > System Tools > System Restore, but reinstalling the driver is definitely the preferrred method).
I can find you the link for the download you'll need - but only if you provide the exact make and model of your computer.