Even experienced bakers have the occasional loaf of bread that is over proofed and does not rise properly in the oven. It doesn't need to go to waste. Instead, slice the loaf and use it to make sourdough croutons, bread pudding, breakfast casserole, or even breadcrumbs. Next time, keep an eye on your loaf during the bulk rise. It may not need as long for proofing as stated in the recipe if your house is very warm, or your starter is very active. Watch for it to nearly double; if it seems to be proofing quickly, you can place the bowl of dough in the fridge to slow the fermentation down until you're ready to shape.