iMac Unexpected Shutdown

I usually leave my late 2010 iMac 21" on 24/7 and put it to sleep when I am going to be away for awhile. This morning, I did not put it to sleep and when I came back about two hours later, the screen was dark, as expected. I tapped the space bar to wake it from its auto sleep and nothing happened. It had turned itself off. Pushed and held the On/Off button and it rebooted and all is right in the world.

Why/how did the machine shut down with no intervention from me?

Perplexed in Florida.
Charlie

You might try checking your error and system event logs to see what happened at the time the machine rebooted.

You can get there by clicking the Apple logo on your menu, then About This Mac, then More Info. A program called System Profiler will then open. On the left side, scroll down until you see Logs (in the Software section). The right-side pane will split, and on top you'll see a list of available logs. Error_log and system.log should (hopefully) contain some information about what happened when your machine went down.

Was there a possibility of a power failure in your house?

- Simon

There are basically three reasons why a Mac would shut down on its own:

1. Power failure.
2. Serious component failure.
3. Overheated.

I think we can discount #2, since it's working properly now. If it was #3, you'd have had to have left it chugging along on something very intense OR the unit is in a very badly-ventilated spot, but either way if that was the cause it will probably happen again at some point, and then you'll know.

(EDIT: just noticed you are in Florida, aka Lightning and Power Flucuation Capital of the US. Makes what I wrote below doubly more likely!)

By far the most likely: #1. Even a momentary blip of a brown-out or surge can cause the machine to shut down. If you have it on a surge protector, check the power-strip's fuse. If you don't have it on a power-strip (or better yet an uninterruptible power supply), you might consider protecting your investment with one.

(EDIT: just noticed you are in Florida, aka Lightning and Power Flucuation Capital of the US. Makes what I wrote below doubly more likely!)

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Good point. Especially around St Pete - right off the left coast of the gulf of Florida. Zaaaaaap! Beautiful area though. ;D

Wouldn't a power failure significant enough to shut down a Mac also be enough to reset a few clocks (and therefore wouldn't have gone unnoticed)?

Shikarnov said:

Wouldn't a power failure significant enough to shut down a Mac also be enough to reset a few clocks (and therefore wouldn't have gone unnoticed)?

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Nope. Computers are VERY VERY sensitive to electric snafus (both too much and too little, which both do equal amounts of damage even though people tend to worry only about the former for some reason).

When I lived in FL, the lights flickering (ie a minor brownout) was sufficient to shut down my iMac. I got a UPS very shortly thereafter.