My brother’s 27 inch monitor on an iMac 2009 started displaying 5 pink vertical lines and wouldn’t progress after the default Apple logon screen.
He went to YouTube for help and found that the solution was to take apart the video card and bake it for 392 degrees Celsius for 8 minutes. This sounded like the same people who would post about exotic tips like charging an iPhone quickly by sticking it in a microwave; or drilling a hole in an iPhone 7 where the audio an auxiliary port is supposed to be. However, my brother was happy to report that it did work and that he now has his iMac back, good as new.
After baking the video card, the heat re-solders the connection and you get a few more years out of your video card after that.