Dell XPS 15-9550
Does the external display work fine?
Run the LCD bist - Turn off the system and press and hold the "D" key + power button to turn on the system and it should run the LCD built in self test - see this link for more info - http://dell.to/1SkXNrG
While you run the LCD test and the below step for diagnostics - swivel the screen and check if the display comes on at any point.
Restart the system and press f12 key on startup - choose diagnostics. Let it run the diags and if you encounter any errors - make a note of the complete error and report the same to us.These are onboard diagnostics and they might possible pick some error if the hardware is not functional.
Download the service manual here - http://dell.to/2d9UV6R - disconnect the adapter and battery and press and hold the power button for 30 seconds and try powering on with the adapter only / battery only and check if the display comes back on.
Reseat the LCD cable and check if possible.
If the system is under warranty, Please click my DELL-username and write me a private conversation with the service tag and your contact details(Name, Phone#, Address and Email). If there is no warranty, then you could contact our team to get a quote for a paid service call - http://dell.to/1vnT6CQ
Let us know if you have any other queries.
There is a function key on almost every notebook to cycle between internal and external display, and another key to cycle the display on and off. By using an external monitor, the computer may not always act correctly after suspend, hibernate or power off, as regards choosing the correct display--internal or external.
So, try using the function keys (i.e. press the blue Fn key + the function key) to straighten it out.