Dell C8618QT User Manual page 57

4. This box also appears during normal system operation, if the video cable becomes
disconnected or damaged.
5. Turn off your display and reconnect the video cable; then turn on both your
computer and the display.
If your display screen remains blank after you use the previous procedure, check your video
controller and computer, because your display is functioning properly.
or
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Troubleshooting | 57