Triage the problem before it triages you

Real emergencies at home are rare, and almost all of them are survivable with the same three moves: get people away from the hazard, cut the supply (gas valve, main switch, stopcock), and only then think about repairs. The reason to read a page like this on a boring day is that during the event nobody reads anything.

Two things are worth doing this week, while nothing is wrong: find your main water stopcock, and check which breaker is the main switch. They are the two answers this tool keeps assuming you have.