What went wrong with my sourdough?
I have to contradict @saj14saj here. I have frequently had bread made with underdeveloped gluten (my grandma uses AP flour and tends to knead very short, 2-3 minutes per hand, and use very short proofing times). The bread is soft and cakelike, but it has no trouble rising, and it is neither flat nor dense. 

On the other hand, I have had bread with exactly the same symptoms as yours - first feeling great, then left out for a long time to proof. After that, it looks good, but one touch makes it collapse into itself. The reason was very clear: overproofing. There is no doubt that underdeveloped gluten cannot have been a factor in my case. First, I am experienced enough to know when my gluten is developed - the bread was kneaded well beyond windowpane test. Second, it was a large batch of dough. I baked the first loaf at the optimal time and it rose just fine and had a nice texture with traceable gluten sheets through the crumb. It was the second loaf, which I baked a few hours later (and the proofing loaf spent them in a 30 degrees celsius kitchen in summer) which made the trouble. The dough had exactly the "loose" feel you describe, unlike the normal, springy feel before the proofing. It collapsed on touch and would not rise at all. It also had a very strong yeast fermentation taste, unlike the other loaf. 

From your description and my experience, my conclusion is that overproofing until your starter died in its own waste products is the most likely culprit. The simple answer would be to not let it sit out overnight. The right amount of time to let it sit would depend on the room temperature, on the amount of starter you used, but also its leavening strength, and that is a bit hard to judge for a newly created starter. My best suggestion is to use trial and error and maybe bake 4 hours after knocking instead of 8 next time, and the time after that adjusting with a smaller increment in the right direction depending on whether the bread turns out overproofed or underrisen.