Black Screen
Whem I shut down my computer sometimes I get a black screen before it goes to login to my iMac. Does anyone else have this problem if ao how to fix it? i am using this version see below, this happens a few times and cannot just get to log in screen. I have unplug both ends of electric cord and held down power button for 40 sec. and then even left it off all nigth to see if it would fix it no luck. Any help would be much appreciated.
iMac, macOS Sierra (10.12.5), Please help asap