PC Stopped Booting Overnight

[Kinpatsu]

As for the title, my pc stopped booting overnight.
All the pieces have been in use for some time already, only difference is that just yesterday I had replaced the motherboard as it was faulty. After that I reinstalled windows, set it up and played a bit before going to sleep. Today I wake up, turn the pc on and as I get to the windows login screen, suddenly the pc shuts down. At first I thought it went to sleep since I had left it idling for a bit (even though I have auto sleep disabled), but as it didn’t respond to any input, I tried to press the power button again, nothing, turn the psu off and back on, nothing, clear the cmos, still nothing. I also have a beeper connected to the motherboard, and it never made any sound

After all that I left the pc alone for about 15 minutes, and pressing the power button again this time led to the pc booting for a split second before turning back off. Jumping the motherboard led to the same thing, the pc just wouldn’t turn on. 
While all this happened, the motherboard rgbs are on like if there were no problem at all. 
To try and test some more, I took the pc apart and first tested just the psu connected to the motherboard and nothing else, and found out that at first it wouldn’t turn on, rgb excluded, but after a bit it powered on (tested both with and without ram installed, don’t think it made a difference). After that I went on and plugged the cpu power to the psu, at first it would turn on for a split second and then back off, then it stopped turning on again and at last it seemed to not only boot but also post. Figuring it was now working, I plugged my gpu back as well, and again the system did post, only to turn off again and go back to square one. 
After that I went and tried a different approach, removing every power cable except for fans and hdd, and shorting the 24pin while not connected to the motherboard did turn them on.

 

Right now I’m thinking that something in the psu might be defective, be it some cable or the whole thing, despite it having worked for almost a year now. I have another psu I can use to test, but it’s going to be several hours before I can. In the meanwhile does anybody have any advice? Thanks in advance

 

PC specs are the following:

CPU: 12700kf (installed with the thermalright bracket)

GPU: 4090

Mobo: ROG Strix z690-a d4

RAM: 2x16gb 3600MT CL16

PSU: Corsair HX1000 (the other one I can use to test is an HX850)

 

[Kinpatsu]

Well, first update, apparently it’s not the psu as nothing happened using the other one. Guess it’s the mobo somehow, despite it having worked just fine yesterday