Hi,
Does the built in screen work? If it does please do try these troubleshooting steps that may help in solving the issue.
- Start by uninstalling/reinstalling the graphics driver.
- Go to Device Manager and uninstall the graphics driver.
- Do also check Programs and Features and uninstall anything related to your graphics card.
- Next is to download and install the latest driver from the manufacturers site.
- Once the latest driver has been installed reboot your laptop.
- After the reboot do test it with your external monitor again, press the Windows key + P at the same time and select between Extended or Duplicate display then press Enter.
- If these doesn't work you may need to try a different cable or try it as well into a different monitor.