Your boiler has lost its connection to the room thermostat or internal temperature sensor, meaning it doesn't know when to turn the heating on or off.

DIY fix possible

Check if your room thermostat or programmer has run out of batteries and replace them if necessary.Ensure the thermostat is turned up higher than the current room temperature.Reset the boiler by turning the mode selector dial to 'Reset' for five seconds and then back to its original position.