How do you rebuild a routine after a period where everything fell apart?

Start with one anchor rather than a full schedule. A fixed wake time is usually the highest-leverage single choice, because it stabilises sleep, light exposure and meal timing downstream. Add one more element only when the first is genuinely automatic, which usually takes a couple of weeks rather than days. Expect it to feel disproportionately hard, since executive function is often reduced during and after difficult periods. Lower the bar to something you would be slightly embarrassed to admit is your target, and let momentum rather than ambition do the work.