We all hope it will never happen to us, but obviously it regularly happens to someone. When water comes through a bedroom ceiling it is usually from a burst pipe or leaking cold water tank in the loft.
Turn off the main stopcock (clockwise). The stopcock can usually be found under the sink, or in the airing cupboard. This will stop any more water entering the cold water tank.
In order to drain the cold water tank as quickly as possible and, thereby, reducing the amount of water available to cause damage, turn on all the taps in the house and flush all toilets. When the taps stop running the leak should stop.
If the water is anywhere near a light fitting or switch, remove the appropriate fuse from the main fuse box.
DO NOT replace the fuse until everything has completely dried out.
If the water is causing the ceiling plaster to bulge, hold a container under the bulge and make a hole in the bulge with a screwdriver or scissors, thus releasing the water. This should confine the damage to a smaller area.
Switch off the water heater or damp down a solid fuel boiler (the fire does not need to be completely extinguished as the hot-water cylinder will not have been emptied) - unless someone takes a bath of course!
Find the source of the leak. If it comes from the tank itself or a pipe which is fed by the cold water tank, fix the ball cock in the tank in order to close the valve (i.e. raise it to a high position) thus preventing the tank from refilling once the water is turned back on.
As long as you are sure the ball cock valve is closed, turn the stop cock back on. This will give you cold water in the kitchen tap (as this comes direct from the mains), toilets cisterns can be filled with a bucket.