While You Wait for the Truck
Shut the main water valve before anything else. If you cannot find it or it will not turn, the curb stop near the street can shut the supply, though it may need a special key. Next head to the panel, but only if you can get there on dry footing.
Water on the floor plus energized outlets is the hazard that hurts people on these calls. If the water has reached outlets, extension cords or electronics, keep out of the room and let the dispatcher know. Get papers, medications and valuables up and off the floor.
Dispatcher tip: Tenant with a landlord who is not answering? Close the valve, photograph everything, and send a timestamped text and email so it is clear you moved fast.