From 10.8 onwards, you will not see the classic grey display, but your Mac will spontaneously restart or shut down. Here are the details, and what to do next.
restart
How to explore what happens during startup, and hone in on problems. The second tutorial, medium level.
A basic tutorial showing how to get useful information from Sierra’s new logs, using LogLogger5.
Sometimes you can’t use the menu command, or it doesn’t work. What do you try next?
Does El Capitan suffer the most evil bug of all, which suppresses the normal kernel panic process?
For the last 15 weeks, this iMac has been freezing sporadically. I have run out of ideas, and hope that 10.11.6 fixes the bug.
Things were not looking good: my Mac would not start up, and the RAID would not shut down. Stalemate.
The Eclectic Light Company 