Question after installing rtx 3080 my sistem will not reboot

Oct 7, 2022
Hi all
This is my first message, i signed up to try to solve a problem.
Today I got a new pny rtx 3080 ordered to replace my old msi gtx 1070.
After turning off the pc, I removed the old vga and put the new one. Although I am not a hardware assembly professional, I was careful not to disconnect or touch cables other than those relating to the power supply of the new board.
Here the first problem arose. The old 1070 had a single 8 pin, while the 3080 has 2. Thinking it was a double slot to choose simply, I did the first test with a single cable and obviously pc didn't boot.
After a quick web search I got that I needed another cable and I recovered it from the psu box. I connected also the second 8 pin cable, but still nothing.
I scrambled to figure it out, I unplugged all the USB devices connected to the mobo, even the hdmi cable, I even removed the power supply connection with the UPS, connecting directly to the electrical outlet. Pc dead.
At that point, i tried to install again the old 1070, but the pc continues to show no signs of life. Then I removed the case plate to access the power supply. I disconnected and reattached all the sockets (mobo, cpu, pci, etc.). Still nothing. Also tested without any vga, zero. It just doesn't turn on. And I hadn't had any problems before, I used the pc every day to work and occasionally fly on msfs
Do you have any ideas?

One more thing: when I connect psu to the wall plug, the power led of the mobo - notthe others (vga, boot,dram,cpu) lights up regularly

My configuration is :

Mobo Asus Tuf z370 pro gaming
Cpu i7 8700k
850w psu silverstone st85f-pt
Ram x4 ddr4 corsair vengeance 8gb 3000 mhz

Thanks in advance for the support