Hi, yes that group was invisible because it was an earlier draft I wanted out of the way. You can see in the original post that I have an entire visible group I was working on that had disappeared, so the invisible group is not relevant to the question at hand.
What you're showing here is the exact actions I took -- just made a little square with the Artboard Tool. Except when I undid the action, the "Artboard Layer" disappeared, but nothing else changed. So, it was stuck in that state with no Artboard Layer and no way to change anything, even if I went back 20+ steps. Nothing I did put the canvas back to normal.
Apologies if this happens to anyone else who finds this question looking for an answer. I ended up just deleting the whole project. One single click is not worth 2 hours of pulling my hair out and crying and scouring the internet, and it's ridiculous that I would even have to do that much in the first place. Luckily this was just a quick little illustration, but what if this had been a much bigger time intensive project or I was on a deadline? I'm not a novice by any means, and I'm not at all happy with this software.