Unplug the computer, open the cover, and reseat the video card (pull it out a bit, then make sure it fits snugly in its slot). If dusty, blow it out with compressed air. Someone brought me a computer just last week with the exact same symptom description (they had tried 2 monitors) and it turned out to be a loose video card. If that doesn't work and you can hear the hard drive whirring, I'd suspect the video card has gone bad, which can happen without warning.
Good luck.