After an Apple Sierra update last night, the 5K Mac that my parents use was stuck at a black screen with just the mouse cursor. Thanks Apple!
The solution to this I found here: https://community.spiceworks.com/how_to/82502-mac-mac-boots-to-a-black-screen-with-cursor
Keep in mind that you are in a terminal type interface and if on the 5K iMac then the text is readable by ants only. But it worked for me with some major eyeball focusing. I could never have expected my folks to do this.
Also, point 3 may take a few seconds to run so do not rush ahead.