1. Check your laptop for any hardware or video card problems.
2. Check the video cable connection between the laptop and the external monitor.
3. Check the actual resolution setting on the external monitor.
4. Go into your laptop’s settings and adjust the resolution to match that of the external monitor.
5. If the laptop settings are not compatible with that of the external monitor, try reducing the resolution on the external monitor.
6. If none of these steps work, bring in your laptop and external monitor to a repair shop or service center for further diagnosis.