Okay a lot going on here and not really a lot of information

First thing I would check is does your motherboard have error lights? Even my computer thats 20 years old has lights that flash in patterns as booting. You use this to find out which part of the booting sequence failed, power, ram, video card, CPU etc. Find out which motherboard you have and find the owners manual. If you have SLI available it's an expensive motherboard and most likely has this feature.

Second, double check everything is plugged into the motherboard, ram all cables etc. Something may have shaken loose?

Might be worth physically inspecting the video card. That you could boot up once makes me think this failed. Maybe a capacitor on the video cap exploded or got knocked off.

Lastly try and take the 1080 out of your girlfriends computer and see if you can boot with that. This would help narrow down issues?

One more thing check if you have an igpu. Something may have failed and the motherboard switched to on board graphics. They'll be extra vga/dvi/HDMI ports on the back. Your lack of video could just be from having your display plugged into the wrong port. So try to hook up into any video out put there.

Also perplexity, chat gpt and Gemini can be helpful in trouble shooting.

Godspeed