A switch is easy enough to check IF YOU ARE COMPETENT.
If you had cause to do anything with the wiring you may have put things back incorrectly. Easy enough to check IF YOU ARE COMPETENT.
The on/off switch is part of the PCB. You may have not properly engaged the actuator on the back of the knob with the switch on the PCB
Manual available here: https://www.dhsspares.co.uk/userfiles/Boiler_Manuals/ART416.pdf
Get someone who knows what they are doing and understands what they are doing, to look at it.
XXX
December 2016