When your kitchen faucet won't turn on, try a faucet in another room. If your bathtub faucet won't turn on either, or you find any other tap that won't turn on, you've got a system-wide problem with the water supply. If only the kitchen faucet is affected, however, you can focus your efforts on that fixture.
If you get municipal water and you experience a water shortage, check with neighbors. If they are having the same problem, call the water company, which may be doing work that requires a water shutoff. If your house is on a well, check the well and pressure pumps. The problem could be something as simple as a blown breaker.
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Tap Won't Turn On
If a particular fixture, such as a kitchen faucet, won't turn on, first check the shutoff valves under the sink. Make sure both of these valves have been turned counterclockwise as far as they'll go.
The next thing to check is the faucet aerator because when it gets blocked with scale, it won't allow water to flow. You can sometimes unscrew aerators with your fingers but one with a lot of scale will probably be stuck, so you'll need adjustable pliers. Once it's off, turn on the faucet. If water flows, soak the aerator overnight in vinegar to clean it.
Scale can also collect inside the faucet, but it's rare for the deposits to completely block water; usually a tiny bit can get through. To check the valves, turn off the water shutoffs, remove the faucet handles and unscrew and remove the valves. If they are indeed blocked with scale, you will probably need a valve puller to get them out so you can clean them in vinegar.
Hot, Cold or Both?
You might notice that none of the hot water faucets in the house are working, but the cold ones are. That points to a problem with the water heater. If yours is older than 15 years, the problem could be that it's full of rust and scale. It needs to be flushed and it needs a new anode rod, or you need an entirely new water heater.
Older houses with galvanized plumbing can experience loss of water pressure because of corrosion in the pipes, and this could become so severe that neither the hot nor cold water at some faucets will work at all. There's no easy fix. It's time to replace the pipes.
Leak in Water Supply Line
When there's no water coming from a particular fixture, and you can't find the reason, the problem could be a leak in the water supply line to that fixture. A leak is a catastrophic event that is usually easy to spot, but sometimes the pipes run underground or under the house and out of view. On the other hand, if the pipes are in the walls, you'll have no trouble noticing the water on the floor, the soaked drywall and the dripping.
If you have a leak, you need to shut off the water as soon as possible because the damage done by leaking water increases the longer the water runs. Your best bet is to turn off the water to the entire house until you figure out what to do, which will probably be to call a plumber.