Computer shutdown on its own

I have a Vista Hoe Premium 32 bit with Vista SP 2

 

I know this issue has nothing to do with if my NIS is functioning properly or not.

 

Yesterday evening when I came home, my PC was sut off. Nobody was home so I know that it was not shut offby Human hands.

 

Now I know that if we have even the briefest power outage, the PC will shut down and the microwave clock will set to "0:00" on the time. However, the microwave closck was not changed this time.

The router was on and the printer was on. But the PC was not (power button blue light not on)

So I go to turn on the power button to the PC and nothing happens. No power. Great I think, its finally broken. Then I decided to unplug the PC power cord from the surge protector strip. Sadly I only waited about 10 seconds before plugging it back in. I remembered too late that one should wait at least a few minutes before plugging it back in. So like I say I plug it back in within a few seconds. Then I hit the power button on the PC tower and the blue light comes on.

 

So I'm perplexed why the power did no turn back on when pressing the power button, but did come on after unplugging/replugging the power cord.

 

Is this not that unusual to happen some times?