Plug in a different monitor and review the display settings to see if standby mode was somehow switched to always instead of 20min. or never. Otherwise the monitor itself may need repair or replacement from a defect.
Those would be the two items off the top to consider there. If there is no problems found with the display settings in Windows the monitor has a switching problem and is not going into the normal display mode once a signal is present. A third item of course is simply no signal being seen coming from the video card itself.