PC won't turn on anymore, but there's a green light on the motherboard.

[elpinks]

Earlier today my PC was working fine, but after shutting it off to go run an errand, I came back to find that it would no longer turn on. I don't think any connections or components were moved around, and I've already been checking the connections anyhow but haven't found anything.
I'm not sure what the motherboard is, it's the one that came with the computer when I bought it. It's an ASUS with four RAM slots, I know that.
It has
Intel i7 CPU
Sapphire AMD R9 280x GPU
EVGA 850W PSU
32 gb RAM (four 8gb sticks)

If it's of any importance (and I suspect it might be) my PC was doing this odd thing where I'd put it to sleep, but when trying to wake it up, it would shut off instead and refuse to start up for a few minutes before going back to working order as if nothing happened. This happened a handful of times within the last two months.

 

[profoundnoah]

Is your PC Air or Liquid Cooled? If it's liquid cooled, then there could be a leak. If not, then follow these steps to diagnose your issue. http://www.tomshardware.com/forum/261145-31-perform-steps-posting-post-boot-video-problems

It's just a conjecture, but I think that it could be a potential PSU or board issue, which is rare but not unlikely for ASUS and EVGA.

 

[elpinks]

The computer is air cooled and I've already gone through that diagnostic without any noticeable change or success. However, something happened to my computer overnight that has left me confused.

Before going to bed, I unplugged the PSU from the wall socket and left it that way. This morning, I plugged it back in... and it started up. I had mentioned this had happened before but only after putting the computer to sleep and the unresponsiveness would only last a few minutes.
So either my problem is somehing else, or the power of wishful thinking is stronger than I thought.
I guess now my concern is figuring out why and keeping this from happening again.

 

[profoundnoah]

Alright then. Just covering all bases, have you tried using a different outlet? Be sure to tell me if the issue persists.

 

[elpinks]

I tried changing the outlet back when it wouldn't work.
Right now it seems to be working fine. While this solves the problem in the short term, I gt the feeling that I haven't seen the end of this issue.
I'll keep an eye on it and see if I can identify a cause for this.

 

[elpinks]

Alright, it happened again. Just like last time. It worked fine while I used it, then it was turned off, and when I came back, it wouldn't turn on again. I tried out what worked last time and disconnected the power cable from the PSU and waited. Waited about an hour and tried it again, still nothing. No surprise there.
Last time, doing this overnight worked for some reason. What could be happening that would cause the system to work after being cut off from power for a while?
I should also note that this happening twice in a row is something new. When it rarely happened in the past, it was only every couple of days or weeks. Now it's every time the computer is turned off.