It's not unusual for a device driver to become corrupted or just 'disappear' from Windows from time to time, and sound drivers are no exception.
So re-installing the sound driver may be all that's required.
However, if your sound device is built on to the motherboard (also known as 'integrated sound') then you can get the driver from the motherboard manufacturer. MSI has a "Live Update" service much like Microsoft Update here:
http://global.msi.eu/index.php?func=downloadindex
However, if your PC is a "big name" brand like Packard Bell, HP, Compaq, Sony, Gateway, Dell etc., you should use their websites instead for drivers. This is because these companies often install custom-made drivers for their motherboard components so 'standard' drivers from the mobo manufacturer may not work.
If a driver re-install does not fix it, remember that components do not last forever. The sound chip itself may have failed. If it has you'll have to get a new PCI soundcard.