Chosen Solution
internal and external display is not working (blank screen), but the laptop is turning on. what is the problem ?
The first thing I would do is disconnect the external display (in fact, disconnect everything except the charger cord) to to simplify the diagnostic process. If it starts working with the built-in screen then you know the problem rests with something you unplugged. If it does not start working it is a safe bet that the computer is not getting through the boot-up process. There are numerous possible reasons for this, such as bad RAM or corrupted PRAM. Does the computer ‘chime’ when you power it up? But first try resetting the PRAM. To do this start at ‘power off’. Press the power button like you are starting it up, but immediately press and hold four keys on the keyboard…. Command (four leaf clover), Option (alt), P and R. after a few seconds you SHOULD hear the computer ‘chime’. Don’t let up up the keys on the keyboard yet! Keep holing them down until the computer chimes at least two times, preferably three times. Now, release the keys and hopefully the computer will start up. If this worked it means that the PRAM got corrupted. Why? How? I can’t tell you. Another possible source of the problem could be the SMC (System Management Controller). Go HERE ==> https://support.apple.com/en-us/HT201295 to learn more about that. But, hopefully the PRAM reset solved the problem. At least that’s where to start.
