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 positioned on the lowest floor of your home.Place a cloth under the radiator bleed valve and open it slightly to drain a small amount of water.Monitor the boiler's pressure gauge until it returns to the green zone (typically between 1.0 and 1.5 bar) and then close the valve.