Press the FN F4 key, which toggles between the LCD screen and the external LCD, you should get the intenal, the external and then both screens. Some times the switch to turn off the internal screen hangs so open and close the laptop. The graphics on the laptop works. There were some issues with the NVidia graphics failing a few years ago due a defect. Check the HP site to check it your computer was among the ones effected. The video cable of the invertor fot the backlighting may have failed, unless you have a LED back lighting. The the invertor is not expensive, but its' replacement requires that the laptop to be disassembled.
It's not easy to get to, but I replaced the CPU on mine and it worked fine.
I had the no display, no post, no boot issue. (Of course, there's always that 10% chance that all it was was a bad connection and taking it apart and putting it back together was really the solution. After I test the CPU in question with another AMD laptop, I'll post back.)
EDIT: I'm sorry, this information is not relative to you as you were able to post, boot and view on an external monitor. In which case, you would need to put the laptop under a bright light and let us know if you can see any faint video on the screen.