First let me welcome you to sevenforums. there are plenty of resources and people to help you so dont hesitate to ask.
There are several reasons your sound stopped working.
It could be turned off in control panel (does the speaker in the system try have a read circle over it?), it could be a bad audio driver
(try re-installing it) If you could give us some more information about your system it would help. Is it 32 bit or 64, do you have a sound card, is it a laptop or desktop, etc
Hope this helps