|Is it plugged into a video card on the computer or onto the video port that is part of the motherboard (onboard video)?|
If a video card it is possible that the card is no longer into its motherboard slot properly (could have been jarred out of whack when you connected cable to monitor or moved the PC or something).
If it is a video card, shut down PC and unplug it. Then open the case and remove the video card (you may have to unplug cable to monitor) and then replace it back into its slot). Close case and re-attach monitor cable. Plug in and power up.