While You Wait for the Truck
Close the main shut-off first. 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. Then go to the breaker panel, but only if you can reach it without standing in water.
Standing water and live outlets are the biggest danger 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.