PC won't boot into BIOS

[Cleptomanier]

(I have an old thread with another Problem from the same PC(except the CPU), so you can find more background to the PC here)

 

So I've build my first custom rig and, after getting a new CPU because the old one seemed broken, I powered it up. Everything seemed to go well for the first 10-15 seconds, the fans started spinning, lights were on etc. But then the debug-light 1(CPU) blinked and the PC shut itself down. The screen doesn't show anything, USB ports do not output power. After a few seconds the PC tries to boot again, with the same result. This cycle seems to continue indefinetly.

My troubelshooting steps were:

Pluging the PSU in and out of everything

Reseating CPU, RAM, GPU and SSD

Unplugging every not-necessary conection from Motherboard and PSU

Replugging everything I could

 

I think it might have something to do with the PSU because I had to plug it in and out many times during my first problem, and it makes some unsetteling electrical sounds if I plug it in. They aren't that noticeable and I can't tell I they were there from the begining, maybe I just didn't notice them.

 

My specs:

CPU: Intel Core i7-9700k

GPU: MSI Nvidia RTX 2060 Venture

Motherboard: MSI Z370-a Pro

PSU: CoolerMaster masterwatt 550w

Case: Sharkoon TG5 RGB(I'm using the provided coolers)

SSD: 512gb adata gammix s11 pro

RAM: 2x8GB G.Skill AEGIS F4-3000C DDR4
CPU cooler: Be quiet! BK009

HDD: WD Blue 1TB SATA 6Gb/s HDD internal 8,9cm 3,5Zoll

[Suppersupport]

Try the mobo next I guess.