Your boiler has lost its connection to the remote thermostat or controller, which prevents it from knowing when to turn the heating on.

DIY fix possible

Check if the batteries in your wireless room thermostat are flat and replace them if necessaryEnsure the room thermostat is within range of the boiler and has not been moved to a spot where the signal is blockedPower cycle the boiler by switching it off at the fused spur for 60 seconds and then back on to force a reconnection