Windows has recovered from unexpected shutdown

Imagine the following scenarios.

• After leaving your computer for a while, you come back and find out your computer is turned off. 
• You put your computer into sleep, but the computer is turned off itself.

When the computer is turned on, it tells

Windows has recovered from unexpected shutdown.

After following the advices from Internet, you still cannot figure out why your computer shuts down itself. If you are sure the hardware is fine, and use Firefox, then disable the automatic update of the Firefox and Firefox add-ons. This may solve the issue. Another you should avoid is, if you just wake up your computer from hibernation, don't go to back hibernation in a short period of time. I noticed that this is a cause too.

Update: if the approach above doesn't work for you, try to change the anti-virus software in your system.