The water is being shut off, or has been.

Before it goes: fill drinking containers, fill the bath for flushing, and note where your main shut-off valve is. When it goes: switch the water heater off if there is any chance the tank will drain. When it comes back: open the lowest cold faucet first and let the air out before anyone runs a shower.

Most of the trouble a shut-off causes happens in the hour after it comes back, not while it is off.

Before it goes off

• Drinking water, in clean containers. Roughly a gallon per person per day is the standard planning figure, and more in heat.
• The bath, or buckets, for flushing toilets. A toilet flushes if you pour a bucket of water straight into the bowl fast.
• Wash up and run the dishwasher and washing machine beforehand rather than after.
• Fill the kettle and a couple of pans.
• Know where your main shut-off is, and check it turns, because a planned outage is a good moment to discover it does not.
• Tell anyone in the house, especially anyone who might start a load of laundry.

While it is off

• Do not run the dishwasher or washing machine, which will fill partially and stop.
• Turn off the ice maker and any humidifier plumbed into the supply.
• Water heater off, as above.
• Do not leave faucets open and unattended. When the supply returns it does so without warning and an open faucet over a blocked drain floods a room. Open one deliberately, in a sink you can watch.

When it comes back

1. Bleed the air. Open the lowest cold faucet in the house first, let it run until the spitting and banging stop, then work upward and outward through the house. Air in the system is normal and it sounds alarming.

2. Expect discoloration, because a supply that has been off and back on stirs up sediment. Flush cold faucets only, and do not do laundry until it clears.

3. Follow any boil water notice exactly, and assume one applies after a mains break until told otherwise. It covers drinking, cooking, ice, teeth brushing, and pets.

4. Walk the house and look for leaks. This is the step people skip and it is the one that matters. Pressure returning to a system finds any weakness at once, so check under sinks, behind the toilet, at the water heater, in the basement or crawl space, and at ceilings below bathrooms.

5. Water heater back on, only once a hot faucet runs steadily.

6. Clean the aerators a day later, since they will have caught the debris. Otherwise it turns up as a pressure complaint next week.

If it went off without warning

Then it is either the utility or your own supply, and telling them apart is a two minute job. Second faucet, then the neighbors, then the main valve.

If it was cut for non-payment, do not open the meter box valve yourself. Utilities treat that seriously and it can carry a penalty on top of the debt. Most run hardship and payment plan schemes that are not advertised, and calling is usually better than it feels.

If the shut-off is long

A day or more changes the calculation:

• In freezing weather, drain the system rather than leaving it full. Open the lowest faucets and let it empty, because a full static system in an unheated house is how pipes split. The frozen pipe sequence.
• Think about whether the house is habitable. No water and no sanitation is a genuine habitability question, and if you have to leave, keep the receipts. Additional living expenses.
• If you rent, report it in writing. Loss of water supply is an emergency repair obligation in most places, though not when the cause is the utility. How responsibility divides.

The short version

Fill containers and the bath before it goes. Switch the water heater off if the tank may empty, and back on only when a hot faucet runs steadily. Bleed air from the lowest faucet upward when it returns, expect discolored water, and then walk the house looking for the leak that pressure has just found.