1 (cliche) Have you turned the computer itself off and on again?
IF that worked, disable the sleep mode. (control panel, power settings)
2: if #1 didnt work, and you still are not getting any display, Check all connections and make sure everything is plugged in (i know cliche again)
3. If you get a display, try to update all of your drivers. (Windows update usually does the trick.)
3. IF 1,2, or 3 didn't fix the problem, there is a fairly high chance that you have a hardware issue. It could either be a RAM issue (bad sticks) or a bad or glitchy video card.
Pm me if none of these work, I can write up a more detailed diagnostic for you.