The following paragraph refers to Windows XP. (There might be minor differences with other versions of Windows):
Go to the Control Panel.
Double-click 'Sounds and Audio Devices'
With the 'Volume' tab selected, check that there's NOT a tick alongside 'Muted'
(If there is, click to remove it, click 'Apply' and 'OK').
If the sound is not muted it's likely that Windows has lost the information relating to the driver for your sound card (or 'on-board' sound, as appropriate). There are two possible solutions:
1 (Guaranteed to work but possibly tricky to find the correct file to download):
Download the driver set-up file, double click to install the driver and restart your computer.
2 (Very likely to work but possibly not guaranteed to do so):
Run System Restore (from All Programs > Accessories > System Tools) to take your computer's registry back to a date before the problem first occurred.
I'd try method 2 first. (If that doesn't work we'll need to know the make and model of your soundcard and/or your motherboard and/or your computer in order to locate the relevant river file).
Chris