My PC shuts down on its own?

Hi all! To start I take care of my PC like it's my own child. But lately (it's only happened twice), my PC has shut down on its own or it has went to sleep on its own even though I was still using the PC.

What's weird is that when the PC shuts down on its own, it does so in a very "normal" manner. "Normal" as in: just like if we were to manually shut down the PC through the start menu.

I'm not too sure as why it does this as I believe that the hardware shouldn't be causing the whole rig to power down in this manner. Any thoughts?

Thanks!