Black screen on launch

Hard-booting did it for me (hold down the power button until the machine shuts down, then restart it)

E: this happened after the installation process.

i tried it. also tried Safe Mode. Did not help.

I found the solution!

First turn off your computer, next hold down command+option+P+R, and while you are holding it, press the power button once, keep holding down on until you hear the start up sound. Done!