PC worked perfect yesterday, now won't turn on.

Gaming PC was working perfectly yesterday, played games, turned it off, went to bed, this morning, the power button doesn't turn the PC on.

Even weirder, this is the 2nd time this has happened. The first time the PC wouldn't power on, I assumed my PSU had died (even though it was a good quality one), so I replaced it, rebuilt my PC with the new PSU and brand new power cables and it came back to life, back to business.

A month or so later, it has now happened again. I see lights on the MOBO so the PSU must be alive but the power button doesn't do anything. I have tested multiple power buttons and nothing, I have double checked the power buttons work on other PC's, they do.

If it's not the power button or the PSU, I have no idea why my pc just won't turn on after X amount of time. RAM is brand new, CPU works, GPU works, everything was running smoothly just yesterday, now nothing..

Any help would be much appreciated.

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UPDATE:

My PC just came back to life all on its own, it was honestly kindof spooky. I wasn't near it, I didn't hit the power button, it just.. turned on. Does anyone know how that's possible? I think it might have been related to capacitor discharge. I performed one and then walked away for a few minutes and it weirdly came back to life.