Secondary monitor shuts off when laptop lid close

By default, most laptops will go to sleep when you close the lid.

1. Click the Start (Windows) button
2. type "Power Options" and press enter
3. In the left hand pane, click "Choose what closing the lid does"
4. Under "When I close the lid:" select "Do nothing" (you can do this for on battery, plugged in, or both)
5. Click "Save Changes"

Now when you close the lid, your laptop should keep running and should recognize that only the external display is available.

Try selecting the external monitor as a primary display. If that doesn't work check if there is a button to switch display outputs on your laptop.