I recently faced the problem where my Windows machine would randomly shutdown. No warning, no messages!

However it would consistently shutdown everytime randomly.
Interestingly using the power button wouldn't restart it either. I would have to take out the power cord and plug it back in after few seconds. Then the power button would function normally.
I suspected SMPS as the culprit, specially because it required me to unplug the power and plug it back again before working. So I changed the SMPS couple of hours ago.
So far it looks like my hunch is correct.