The order matters more than the speed
Close the valve before you dial anything. Almost every supply failure in a house is still delivering water at full pressure while the owner is on the phone describing it, and a few gallons a minute across an hour is the difference between a room and a floor. The main shutoff is usually where the service line enters the building, and if the interior valve will not turn, the meter box at the curb is the fallback. On a well there is no curb stop at all: the pump breaker and the pressure tank are the shutoff, and both are the owner's.
Then the electricity, if water has reached anything electrical. Kill the affected circuits at the panel rather than reaching for a switch in a wet room. If the panel itself is in the wet area, or if water is standing anywhere near live equipment, leave it alone and let the utility or the fire department handle it. Nothing in a flooded house is worth a shock, and this is the one point in the hour where the right move is to stop and wait for somebody else.
After that, and only after that, make the calls: the utility if the leak is on their side of the meter, then a restoration contractor. Somewhere in the same hour, start taking photographs. Every wet room, every affected item, the source itself if you can see it, and a wide shot of each room before anything is moved. That record is worth more later than any of it feels now, and it takes four minutes.
On a slab, the water goes sideways and shows you nothing
Newer suburban construction across this region is slab-on-grade, and it has been for decades in the growth belts east and south of Memphis and around every county seat that has expanded since 1985. A slab gives water no path downward. A failure at a supply fitting spreads out flat under the finish floor instead, traveling under a floating floor or laminate for the length of a hallway while the surface above it still looks entirely normal.
So on a slab, the wet room is frequently not the room with the failure. Water from a water heater in an attached garage crosses under the common wall into a bedroom, and the first thing anybody notices is a swollen baseboard two rooms from the origin. Grout that cracks along a hairline, a warm patch on the floor, a floating floor that has started to telegraph its seams, a meter that keeps turning with every fixture closed: those are slab symptoms, and they arrive days after the water did.
The practical instruction that follows is to stop trying to work out how far it went by looking. You cannot see it, and pulling up flooring to find out usually destroys more than it reveals. What answers it is a moisture meter run across the whole floor plate, which is a fifteen-minute exercise for somebody who does it every week.
Over a crawl space, it goes down and comes back as humidity
The older stock in the county-seat towns sits over crawl spaces, and there the same failure behaves in the opposite way. Water leaves the living space through the subfloor and lands on the ground under the house, where it is out of sight and, as far as the owner can tell, gone. The floor dries, the ceiling below has no ceiling, and everybody moves on.
It comes back as vapor. Ground under a house in this region stays cool through a hot summer, so a crawl space that is already the coolest surface in the building now has standing water in it as well. That water evaporates upward into the joists and the underside of the subfloor, and what began as a fifteen-minute plumbing failure in June turns into a call about a smell in August. Insulation batts between the joists hold the water against the wood and make it worse rather than recording it.
Which means the crawl space gets opened and read on the same visit as the living space, not on a later one. A meter on the joists, the rim and the underside of the subfloor, and a hygrometer left down there overnight, will tell you whether the water actually left the building or simply changed address.
Two or three things not worth attempting
Do not go looking for the leak inside the wall. Cutting exploratory holes through wallboard to find a pipe is how a small repair becomes a large one, and a meter finds the wet cavity from the outside without opening anything. If a wall genuinely has to come open, it should come open in a defined section at the framing, which is a decision to make with readings rather than with a utility knife and a guess.
Do not try to dry a house with box fans and open windows in a West Tennessee summer. Outdoor air here sits above the EPA's 60 percent ceiling for controlling indoor mold for months at a stretch, so pulling it through a wet building is importing more water than it removes. It feels productive and it is the wrong direction. Air movement is only useful when it is paired with dehumidification that takes the water out of the air, and in July that pairing is not optional.
And do not lift and reuse wet carpet pad, or push a shop vacuum through a wall cavity, or run the air handler to move air through a house that has taken on water. The last one distributes moisture and whatever came with it through every room the system serves, including rooms that stayed dry. Turning the system off is free and it is usually the right call.
What is worth doing while you wait
Get standing water off hard floors if you can do it safely, and get contents up off wet carpet: books, boxes, anything with a paper or fabric base, and the legs of wooden furniture, which will bleed stain into a carpet within hours. Aluminum foil or a block under a furniture leg costs nothing and prevents a mark that will not come out.
Write down when it started, when you closed the valve, and what you found where. Keep the receipt for anything you buy in the first day. If the water came from a sewer or from outside the building rather than from a supply line, say so on the call, because that changes the category of the water and therefore what can be dried and what has to leave the building. Guessing on that point is not something an owner needs to do; describing honestly what you saw is enough.