Are you able to turn off the water supply to the affected area? Doing this quickly will help to prevent further damage to your home and belongings and neighbouring properties. If you live in a flat this is essential to prevent water damage to neighbouring properties.
There is usually a stopcock underneath your sink or in a cupboard under your stairs. For toilets or bathroom sinks, there is an isolation valve (which looks like a slot-headed screw) which can be turned off using a screwdriver.
If the leak is near any electrical sockets or switches, then turn off the electricity at the consumer unit.
Switch off the immersion or the central heating system.
Turn on all taps to drain the water from the system.
If you are unable to turn off the water, put a towel or container underneath the leak to collect the water.