2009 iMac screen black after drive replacement, help!

I followed this iFixit guide to replace the HD with a Samsung 850 EVO SSD (using a NewerTech AdaptaDrive bracket):

Intel iMac 27" EMC 2309 und 2374 Festplatte wechseln

While the guide says to disconnect all the display cables and lift off the display before replacing the drive, I only had to pull out the vertical sync cable, which gave enough space to remove/replace the drive while propping the display at about 45 degrees. I did NOT unplug the display data cable, the backlight power cable, or the display thermal sensor cable. I also checked that all those cables were still connected and not pulled out accidentally. The vertical sync cable seems to be replugged successfully.

But when plugging in the iMac afterwards, the screen is just completely black, no light at all.

Which cable is the likely culprit? Or is there something else that I might have tweaked accidentally? I deliberately did not unplug all these cables to prevent this kind of problem to begin with so it's quite upsetting. I've seen several posts saying that the screen should work even without the vertical sync cable plugged in, so I don't think that's the problem, and that's the only cable I unplugged.

Any advice much appreciated!