I've just checked the PSU with a known good PC, it works. And PSU from the other PC with mine,not working. The two psu's and the motherboards are exactly the same.
I tried to turn on my PC and it failed to turn on. Did the same after a day, it turned on and started working normally after some try. Tried to check if the problem is solved by turning it off and then back on, no reaponse like before. It has been a few days and no change, no response...