Apple MacBook Pro display shows black screen
Solution 1:
Unfortunatly, it looks like a hardware issue.
Use the external monitor to see what you are doing and go to the system preferences and check the settings. Check the monitor preferences and see if it listed. Try turning the brightness all the way up.
If it is a 2015 model and less than 1 year old Apple should repair it for free. If you have AppleCare extended warranty they will repair it for 3 years for free. Take it to an Apple Store or an Apple approved repair vendor.