Your boiler has detected a communication problem with your external thermostat or smart controls, meaning it cannot receive the command to turn on.

DIY fix possible

Check if your wireless thermostat has run out of batteries and replace them.Ensure the thermostat receiver (usually near the boiler) is switched on and showing lights.Try resetting the boiler by turning the selector knob to 'R' for five seconds.Check if your internet router is on if you use smart controls like Hive or Nest.