When I got up last Sunday morning, I tried to turn on my mac pro which is a 2011 model. That is relatively old, but that is still in good shape. On that morning, it got me a black screen. It has a startup sound. I tried to press the keys randomly. It went to a forgotten password screen. I followed the instructions to reset my password. That still was not working. After a restart, that was still a black screen. Finally, I tried to use Control+Option+Command+Power, trying to reset NVRAM. It accomplishes this times. I believe for some reasons, the NVRAM saved the setting to dim the screen to a black screen.