I have had a 3 way valve motor fail on me in the past that stopped the system working.
Boilers are portrayed to be some sort of magical unit that a chosen few must touch - mine was getting noisier earlier this year, funds being tight I did a bit of research and discovered they are just like a smaller heat exchanger unit I had worked on before in the mob - the noise was coming from the exhaust fan which suffer from dry bearing shafts which stops the fan - no fan, no flow, no flow no fire up.
One whistle around with the vaccuum cleaner a blast of BG Products penetrating oil and all is peachy :P