What does the BIOS settings say? Does your BIOS have an option to disable sound? Also is your OS forcing a mute of sound? (In XP, depending on your driver, it might look a little like this on your bottom right toolbar:
)
One final possibility is simply the onboard sound went bad. It can happen like any other part of a mobo going bad. If it's a new mobo, or the system is under warranty, get it replaced. Otherwise I'd consider buying up a new mobo since it may be getting aged/damaged. A damaged mobo can potentially damage the other components of the system in the long run.
A side option is to buy a simple sound card and have it installed.
But do exhaust the possibility that you have a bad/wrong driver for your onboard sound. Try to find out what mobo you have on your system and hopefully researching that on the internet (mobo manufacturer being the logical starting point) will get you what sound chipset you have.
Frequent chipsets are Realtek (usually AC97), Intel High Definition audio, but there's many proprietary ones out there if you're using a pre-built system, at which case start looking with the system builder first (E.g. HP, Dell, Compaq, Gateway, etc...)
