Please follow below instructions to do troubleshooting.
1. Make sure the pins on the video cable are not bent or damaged.
2. Make sure the connections on both ends of the cable are secure.
3. If you are using an adapter check to see that the adapter connections are secure, disconnect and reconnect if possible.
4. Make sure there are no extension cables or switch boxes attached to the monitor.
5. Test a different video cable.
6. Test the unit on a different computer or video source if available.
7. Verify that you have the proper drivers installed for the video card.
8. Check the problem using other resolutions and refresh rates.