PC Turns Off after just 3 seconds of pressing the power button

[KubaBrdej]

I am having a problem with my pc, I gave it up for a repair so I can downgrade to Windows 7 instead of 10, After a couple of days the PC was ready, I got it, the guy told me that it works all good, everything is running well but when I plugged it in, the pc turned on, and off 3 seconds later, I was confused, I took it all apart, unplugged everything step by step to see what was causing the problem and when I unplugged the Processor and booted the PC again, it turn on and didn't turn off, I don't know what could be the problem other than the CPU, or the socket itself. I hope I won't have to replace the Motherboard

 

[fabiodrm]

CPU overheating may be the problem, are the cooler seated correctly? are enough thermal compound?

 

[fabiodrm]

post your configuration, PSU may be the problem too

 

[KubaBrdej]

Yes I have a thermal cooling paste on my CPU, before downgrading the CPU was usually between 20℃ to 35℃, also my PSU is TESLA 500te br