Double check you monitor to video card connection (assuming HDMI here). If you didn't touch any hardware on your computer recently, it's not the likely cause of your problem, but other things can happen to that cable, like a pet got behind there and bumped the cable or something. Look for the easy solutions first. Do you really mean that the power light on the monitor is totally off, or are you just losing the signal to the monitor?
Also check the power plug to the monitor too, on both ends. Loose connections can be a pain to identify.
Failing that, try uninstalling the video card driver using safe mode, and then let Windows install a default driver when you reboot. See if that driver works without the monitor turning on / off. If that works, then try to install the old driver again.
As Albert Einstein said, "I could have done so much more with a Big Al's Computer!".
