How can I fix my boiler?

when i let the air out of my radiator it shut down and is now not turning on. the same problem happened to me the other day and after a day it was fixed autimatically. it's been two days since the radiators and hot water aren't turning on, when i turn the boiler on it starts up but then shuts down after a minute. and the grundfos selectric-the red circular object- makes a loud rumbling noise.
i have a IDEAL CLASSIC NF Fanned Flue Gas Boiler
it is a NF50 natural gas.