The Apple Macbook Pro would turn on, but the screen would not. The screen would stay black. I could tell the computer was on because when I hit the “caps lock” key, the green light on the key would turn on and off.
I fixed this the following way (this is basically resetting the Power Management Unit — PMU):
I disconnected the computer from the power cord, and from all other cables.
I held down the power button for several seconds to turn off the computer.
I let it sit 5 minutes.
I restarted the laptop while holding down the Shift key. This restarted the computer in Safe Mode, with the screen visible again.
After I logged in (in Safe Mode) to make sure everything was ok, I simply restarted the computer the normal way, and the screen behaved normally again.
If that doesn’t work for you, you could try these steps, documented at apple.com: