1. Check the cables between the computer and monitor. This could include HDMI, display port or VGA cables. Make sure that everything is plugged in correctly and securely.
2. Confirm that the monitor is receiving power and is turned on.
3. Check the output settings on your computer. Depending on the operating system, you may have to change the output settings so that the computer recognizes the display device.
4. Check the display settings on your computer. Ensure that the display mode, resolution and refresh rate are correctly set.
5. Update the graphics card driver on your computer. This would allow the computer to recognize the external monitor so that you can switch from the primary screen to an extended display or mirror display.
6. Try connecting the external monitor to another computer to make sure that the monitor is working properly.
7. If all of the above steps don’t work, try exchanging the cables, restarting your computer and updating the operating system.