Most motherboards come with onboard video, but not all. Check the rear of the PC near the usb ports, if there is a video connector on the small panel there (not the video card output) then the motherboard has onboard video. If that's the case then I would definitely try taking out the video card and plugging the monitor right in to the onboard video and test it that way. It's also possible that the video card doesn't have enough power going to it, if the card is a newer gaming type card. The solution to that would be a new power supply. Check for onboard video first imo. That way you can narrow down the possibilities.