So obviously the first thing to do is to check the boiler. I didn't know much about boiler. I kept forgetting how it looks like because a boiler usually sits next to the heater/central unit and I would get the two confused. I shouldn't. Boilers are cylindrical tanks, not hard to miss.
Anyway, watched this very information Boiler Youtube video:
Starter won't work. So I used the placing a bit on a re-settable thermal switch method and it works.
I may need to replace the thermal switch. But I don't know because when I pressed the reset button on the switch, there was no click and I couldn't really push it in much, so apparently, the switch wasn't tripped.