DIAGNOSIS The monitor is powered on but receiving no signal. The primary suspect is the input source or cable.
FIX First, press the joystick on the back to open the OSD. Navigate to System Setup > Input Select. Confirm the input matches your source cable (DisplayPort or HDMI). If correct, power cycle. Turn the monitor off at its power button. Unplug the video cable from both the monitor and PC/console for 30 seconds. Reconnect firmly, ensuring the cable is fully seated. Power the monitor on, then the source.
IF THAT DIDN'T WORK The source GPU driver or output may be faulty. Boot your PC into Safe Mode. If the display works in Safe Mode, update or reinstall your graphics drivers.
HEADS UP The VG27AQ1A has no automatic input detection. You must manually select the correct input in the OSD if you switch sources.