What were you doing before it happened?
Did it happen after a reboot or in the middle of something?
Have you installed, removed or added any software or hardware to your system recently?? (there are some odd conflicts, for example with some virus software packages screwing sound from certain soundcards)
Obviously you've checked the plugs and power to the speakers (you have, haven't you

) but have you plugged the speakers into another audio source to make sure they're still working???
Has one of the kids accidentally pulled out a cable then stuck it back in the wrong socket??
Check the sound properties in system control panel to make sure they haven't been disabled or turned down by something you've done.....
Failing all that (and probably some other simple stuff i've missed), we'll need to get more technical and system specific
