Dell UltraSharp U2518D User Manual page 49

4 This box also appears during normal system operation, if the video cable becomes
disconnected or damaged.
5 Turn off your monitor and reconnect the video cable; then turn on both your
computer and the monitor.
If your monitor screen remains blank after you use the previous procedure, check your video
controller and computer, because your monitor is functioning properly.
or
Troubleshooting | 49