It sounds as if your PC can't find the sound driver. Sometimes simply restarting your PC will fix the problem. Running System Restore, and choosing a restore point earlier than when the problem first occurred, is more likely to be effective.
However the 'proper' way to fix the problem is to reinstall the driver. It should be on the disc which came with the PC (or with the separate sound card if you added one later on). Otherwise you'll need to download it. To do so you'll need to identify your motherboard (unless your sound is provided by a separate sound card) and then go to the manufacturer's website for the driver download. It can be tricky to find out which motherboard you've got but there's plenty of free software to do the job e.g. Belarc:
http://www.belarc.com/free_download.html
Chris