Pc not turning on after shut down

[devildillon7]

So I just built my new pc:

~Corsair obsidian 750D airflow edition
~Aorus Z370 gaming k3 mobo
~i7-8700k
~(temporary until it arrives) - 1x 4gb ddr4 RAM (this is pre owned)
~ aorus gtx 1080ti 11gb
~Corsair h115i Pro hydro cooler
~Samsung SSD
~corsair m. 2
~Thermaltake tough power grand series RGB 850w

So, I built it, and i turned it on without it being plugged into a monitor- It worked for a minute, I noticed it turning itself off and on - I unplugged it to take it upstairs to put it on my monitor and it wouldn't turn on. Weird. Was working a moment ago. Brought it down and checked all the connections, still nothing.

I have tried everything, the plug socket is on, all fuses working. The psu is switched on. All the headers are secured properly. I have tried my old psu. I have literally completely rebuilt it and made sure everything is in correctly. The front panel header connections are correct. There's no cracks or damage to the motherboard. I literally have no idea what it could be.

The Mobo light isn't on when switched on and plugged in. There's no gpu light. Nothing. It's as if no power is getting through, at all. So I personally can only narrow it down to the motherboard being faulty?

 

[1LiquidPC]

Could be the motherboard as you point out. Have you tried "breadboarding" as its called. I would try only connecting the bare minimum hardware to futher narrow things down. Use onboard video, 1 storage drive, ect. Although unlikely I would give the other PSU a try again using this method. If the onboard power standby LEDs on motherboard are showing no signs of life when just the PSU(with both PSUs) turned on, it would seem likely to be the board. To be sure, are you sure nothing is grounded improperly? Same number of screws and motherboard standoffs secured? Should be 9 for an ATX and some mATX boards.