Your boiler has stopped working because it cannot detect a flame, so it has purposefully shut down to prevent gas from building up inside your home.

Technical description

The burner will go out but, after a short pause, the appliance will make 5 attempts to restart sparking for 5 seconds and then 'lock-out'. This occurs if the gas service cock is turned off or if the boiler fails to light after a maximum of 5 attempts.