1. Check display cable connections for loose connections
2. Check the power connection for external monitor
3. Check the settings on the laptop, if it is still set to the external monitor
4. Update or reinstall the display driver and check if the external monitor works
5. Check if the monitor is detected in Device Manager by pressing Windows + R and typing ‘devmgmt.msc’
6. Try a different cable to connect the external monitor to the laptop, if available
7. If possible try connecting a different external monitor to the laptop, if available