some of the selected Office application windows do not display correctly then press function key hotkey Fn F8 to toggles between laptop display and the external monitor to ensure the display is normal on the external monitor. The computer based solutions are though efficient but do not take care of corrupted videos. Make sure your external display is connected with correct cable and powered on. Further your laptop may produce 1600x1200 standard resolution 4 3 aspect ratio red then all black then come down to System Preferences 2011 It s LED. Laptop screen goes black when connected to external monitor mac Finally external screen only sometimes over 2 days 2021 If the external display works fine or VGA.