Your boiler has stopped working because the water pressure inside the heating system has dropped too low.

DIY fix possible

Locate the filling loop, which is a silver braided hose with one or two valves connecting your boiler to the cold mains pipe.Open the valve(s) slowly to allow water into the system until the pressure gauge on the front of the boiler reads between 1.0 and 1.5 bar.Ensure both valves are turned firmly off to prevent the system from over-pressurising.