Laptop doesn't turn on? - 3

Today I flashed my laptop bios chip with a USB flasher. Then I put everything together, just before the final screwing together, I checked whether it would work. It had a picture and started to boot (before there was no picture due to a faulty bios update)

I finished assembling it and suddenly it doesn't come on anymore, as if it doesn't react to the power button or something.

What can be the reason?

It is best to first check that all cables are properly seated. Are the fans turning and any RGB lighting switched on?
Also, is the operating system already installed?

I press the button and nothing happens