This is an automatic ignition boiler and if it fails to light for any reason (no gas) for example then it will go into lock out mode indicated by the lockout light, the one marked with a flame and a line through it the second one next to thecontrol knob. The boiler needs to be reset by pressing the reset button which is the one next to the lock out indicator. then run a hot tap and providing there is now gas available then it should light. You may need to try this two or three times untill the gas comes through to the burner. It will also not light if the pressure has dropped below 0.5 bar and the pressure may need topping up to 1 bar.