Hi,
Please try these troubleshooting steps that may help.
- While your laptop is connected with the external monitor press the Windows key + P at the same time and select between Extended or Duplicate display then press Enter.
- If it doesn't work uninstall/reinstall the graphics card driver.
- Go to Device Manager and uninstall the graphics driver.
- Check Programs and Features and uninstall anything related to your graphics card.
- Once done, download and install the latest driver from ACER Support site.
- Here's the link of ACER's support site:
http/www.acer.com/ac/en/US/content/drivers
- Once installed reboot your laptop for all the changes to take effect.
- After the reboot do test it again and see if it will work or not.