how to fix my PC?

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tl;dr After installing new hdd no picture on screen, not even bios.

I have a pc with a gigabyte z97 sniper motherboard. Up until today it had a 250gb ssd boot disk and a 3tb sata hdd. Today I installed another 3tb sata hdd after which I briefly unscrewed the motherboard to fix a minor issue with the I/O shield then reattached it with the same screws in the same holes. I never actually "removed" the mobo just moved it 1 cm and back with the power OFF.

When I tried booting it would only find one HDD. After maybe 20 attempts of reattaching the cables (and booting to BIOS after each try) it finally found all three drives. But when I tried to boot into the SSD it would say it wasn't a valid boot drive. Restarting without selecting a boot drive would give the blue PC ran into problem:( screen.

I went to another computer to create a windows install usb and when I came back the PC which had not been touched during this time would no longer give BIOS. Fans and lights turn on but nothing on the screen. Keeping one monitor on the GPU while attaching the other straight to mobo did nothing. I checked every cable and even disconnected the HDDs to see if there was a power issue but nothing changed. When I try to boot without RAM sticks instead of the beeping like I expected the PC starts restarting itself every 10 seconds so I'm not sure if that's intended behaviour in an error situation or not. With the RAM sticks attached it will stay ON until I turn it off. Removing the GPU did nothing either.

What could be the problem and what to try next?