You might have hit the screen lock key, have the brightness turned all the way down, or the computer has frozen. To solve these issues, locate the screen lock key and hit it once, the same for the brightness key and just hi it until the screen brightens. Finally, do a force restart of the computer and the screen should be back.

A couple of things could lead to this. First, check to see if you have muted the computer. If this is not the case, then check to see that you have no external sound devices hooked up to the computer. If still nothing is found and no sound is playing, try to update the drivers for the sound card, or re-install/ find another comparable sound driver. If still nothing has worked, than you need to try to replace the sound card.