Your boiler has detected that the water pressure inside the heating system is too high and has shut itself down to prevent damage.

DIY fix possible

Locate a radiator in your home, preferably upstairs.Place a cloth and a bowl under the radiator bleed valve.Slowly open the valve to release water until the pressure gauge on the front of your boiler reads between 1.0 and 1.5 bar.Close the valve tightly and check for leaks.