Why did my computer just turn on by itself?

[cklynx]

I built this computer around October time and have never had a problem with it.
Literally about 20 minutes ago I was chilling in bed and my computer randomly turned on along with my monitor.
I always shut my computer off when ever I am not using it so it was not in Sleep mode or anything. Apart from the theory of a ghost (lol) what else could have caused my computer to randomly switch on? It's never done anything like this before, this is the first time.

 

[cklynx]

+1 for Ghost.

I wouldn't know mate, especially your monitor turning on too...

Well actually i forgot to mention, when my computer turns on it also powers on the monitor so that could explain that for coming on. I'm clueless though as to why my computer randomly decided to turn its self on