Windows Vista without sound!

Hello,
Our windows vista laptop has absolutely no sound. On the sound button, there is a little red X and it says "No audio output device is installed" We've tried everything to get it back but nothing works and when we go into control panel, it says its working fine. When I right-click the button, a little pop-up comes up, saying
Open volume mixer(in gray, meaning I can't use it) playback devices, recording devices, and sounds. We've clicked on everyone of those as well but it seems they are for different things, not the main sound on our computer. We would appreciate help from anyone who knows what the problem is. Please and thank you.