[SOLVED] Sudden issues with computer graphics (cards and integrated)

Sep 19, 2019
Hey guys,

After swapping out the GPU of a PC for the first time, suddenly that PC no longer displays anything. Both GPUs no longer work, nor does integrated graphics. Integrated graphics does however produce this weird green checkerboard over HDMI. Here's what that looks like:

With a card plugged in, unplugging one of the power supply cables will throw a normal-looking message onto the screen asking me to plug in more power.

Any ideas for why this might be the case? It is not my computer which makes this pretty frightening...
Thanks for any answers, guys!

Edit with a few other features:
After shutting down the computer and playing with cables and the GPU, sometimes it boots on its own repeatedly (probably a bootloop we cant see because of the lack of displays).
There is no error code display so I am not sure if it's stuck on boot or working properly.
There are no beeps or boops on boot.