Check no water is running (i.e. bath/shower/taps) and all appliances are switched off. Locate the stop cock (usually located under sink or by utility meters) and turn off any water supply.
Contact Maintenance in the first instance. If you cannot locate the stop cock and we are closed, contact an emergency plumber for advice or Environmental Health, who will arrange to attend to turn off the water. THIS SHOULD ONLY BE DONE IN AN ABSOLUTE EMERGENCY.
N.B In an emergency, a plumber will merely turn off water at the mains. They will not source the problem.