Why did my iMac restart?

This AM when I clicked on my keyboard or mouse to wake up my iMac, she did not respond. Would like to know why not.

After several minutes, actually probably 30-45 seconds, and some random key selections, it became apparent that the computer was in some stage of a very slow Restart. I don’t recall experiencing this before.

Is there a way to determine what caused the restart. I’m not familiar with the logs kept by the computer.

Thanks for any suggestions… Which may include … “have a nice day and don’t worry unless it happens again.” :wink: LOL

Thank you.

You didn’t mention version of macOS X, or any updates, or whether you have automatic updates turned on. There was a recent OS update that was pretty hefty and re-started a few times before finally starting with a “Setting up your Mac” opening. Yes, it was slow, almost like an install.

If you are facile with the Console utility (Applications/Utilities) you can look at the system logs. It won’t be easy, but if you really want to know, get ready for some interesting “log archeology.” The answer you seek will probably be in there.

Could have been a random cosmic ray (or numerous other reasons) that made it freeze. Just be sure to backup regularly (e.g. ensure Time Machine is set to automatically back up).
If it happens again and doesn’t seem to want to boot up try an SMC reset ( https://support.apple.com/en-us/HT201295 )
If it won’t respond you can force it to turn off by pressing and holding the power button for a few seconds. When it boots up it should do a few system checks.
You can also try Apple hardware Diagnostics https://support.apple.com/en-us/HT202731

My bad for omitting info…

I’m running OS X 10.14.6 and do not have auto updates on.

Thanks all three of you for your responses. I’m off to be educated on this.

OP back again.

As suggested by Steve above, I’m looking at system logs in the Console app.

However, it only includes today. Does it archive previous logs elsewhere?

Thank you

My Mac mini sometimes will not light up the monitor, and connecting via screen sharing paints a solid-color screen (usually green). Through trial and error, I’ve discovered that I can force it to do a soft reboot, which sounds something like what you describe.

I connect via ssh in a remote terminal session, run top -o -cpu, pick up the process ID (PID) for WindowServer, quit top, then run kill -9 [pid#]. As soon as I do, the monitor lights up, and a moment later, a log-in screen appears. Yes, it looks somewhat like a reboot; however, there’s no chime, and the log for the mail server that’s been running all the while in the background does not indicate a reboot, so this is something else; I’ve never been able to figure out quite what (console isn’t any help), nor have I found anyone else experiencing a substantially similar problem. Maybe the graphics card crashed?