We had the same problem, mains was off for 3 days and from the time they said it was back on until it reached us, it took about 5 hours.
So if it's mains water you're talking about, you'll have to be patient and it'll flow as soon as the pressure is up. I heard on the radio today that some Co Co's are deliberately reducing water pressure to conserve water.
If you're referring to taps in the house that are fed by tanks in the attic, you may have an airlock. If you have mixer taps and water is flowing through one tap, you can place the palm of your hand over the outlet and open both taps. The water will be forced up the one that has the airlock and should clear it. If you don't have mixer taps, you can use a piece of hose to force water up the pipe.