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

I know it sounds stupid...

But when you cut your GPU power almost in half... You turned settings down a lot right?

That would cause the crash, and sometimes a crash just kinda sticks, no fans, no lights, no output.

You have to flip the switch on the supply and do the hardest of reboots.

Like I said, you probably already did that stuff. But I've seen it happen.

The first thing you should always do is try turning it off and on again

Just to be safe do a clean install of the nvidia drivers. I've never personally ran SLI but who knows what might linger. Or if you really want to 110% it download DDU and the drivers. Reboot into safe mode (hold down shift when you click reboot, then pick the startup options), uninstall the driver, restart again (ideally with network disconnected) and install the nvidia drivers.

The only time DDU has fixed something a clean install didn't was when I was really messing with some settings, but it doesn't hurt to do it that way.

Not sure since SLI is so niche I had to look up to remember what it is, but basic things to dummy check:

-are the various cables connected properly/card seated properly

-have you put the gifted wife-GPU into your system to test to see if that works on its own

-Is the GPU getting the proper power allotment since you reconnected things

-is your power supply failing

-Are there any past configurations you did to the failed card itself to enable SLI that its now expecting to see a second card and failing since its gone, beyond windows config.

-Is there something in the BIOS causing an issue

-is your MOBO failing

-have you given proper tribute to the local land god recently

if you have on board graphics with your cpu I’d connect your monitor to at least get into your BIOS and or Windows to poke around some more.

hard to really give any ideas not knowing the rest of your system but it’s also a possibility with a card that old that it might’ve just finally died. the 1080 is pushing a decade at this point.

I'd start by reseating the GPU. Bricking anything is unlikely. Unless you were generating a lot of static electricity and zapping the components.

Also, you plugged the GPU power in, right?

1. After you power the system on, does pressing the capslock key cause the corresponding light on the keyboard to change states?

2. Once the system is powered on, does the monitor and its backlight stay on? Or does it fall back to a “no signal” mode?