it would be nice to know the following
boiler make & model
gas council number [eg 47-348-??]
from your description "a few hours later boiler stopped working"
sounds like boiler was working for a couple of hours at least after work carried out
that rules out air in gas pipework
is the boiler flashing any fault codes/lights?
cant help without more info
call back the gas fitter who did the work
manwithnoname, April 2012