New Mainboard, CPU & RAM, won't boot

[matthiaslaumann]

I've got a new Mainboard, CPU and RAM. After installing everything, my PC wont boot.
It starts, the case LEDs and RGB stripes are glowing and the fans are running, I can hear my HDDs are running too, but that's all. I can't even get to BIOS menu.
The mainboard's debug LEDs for CPU and RAM are flashing ad I've got flashing error codes, the only one I can read and that appears repeatedly is "55", no memory installed. But Those two are installed correctly, the two RAM bars are firm at DIMMA2 and DIMMB2 the CPU is placed after its markings.
I've got the following system:
MSI Z370 Mainboard
Intel i7-8700 CPU
Corsair Vengeance 2x8 GB DDR4
MSI GTX 1070 GPU
1x SSD as boot device
3x HDDs
Corsair 750i power adapter

Please help me, I have no idea what I did wrong.

 

[J_E_D_70]

Have you double checked that all psu cables are secure on both the psu and mobo?

Try installing only one ram stick in the primary slot. If that doesnt work, switch to the other stick and try.

 

[matthiaslaumann]

I've tried that now, no change. Sadly, I can't read the other error codes on that counter, they are jumping too fast. 55 is the only one that stays long enough, besides from the flashing debug LEDs.

 

[J_E_D_70]

Remove the gpu completely and plug monitor into the mobo. See manual for resetting cmos and do that then boot again.

 

[matthiaslaumann]

I‘ve found the problem, but now I‘ve got another one. A single pin on the CPU socket was broken. Got a new mainboard, the PC is booting now. Finally.
I‘ve wondered why I can‘t establish an internet connection and stumbled to the device manager. It seems several devices, mainboard-related I think, are missing or without drivers. Well, I can‘t install any. The driver disc won‘t do anything.
The disc reader is ok, the keyboard too, but my mouse driver won‘t recognize the mouse itself and, yeah, no internet.