what make and model of boiler is it?
does it have a permanant pilot light i.e. is the pilot light always lit even when the main burner is not lit, you have to press in a grey knob, press the spark button to light the pilot, keep the grey knob held in for 10 secs and then the pilot stays lit. or does the pilot light electronically each time the boiler starts up?
did you let the gas run out before you put more credit on it? quite often a boiler will work fine until you turn the pilot off and then it simply wont re-light, this is usually because the thermocouple was on the verge of failing.
if its not the thermocoule at fault then the pilothead or injector could be blocked and if that doesnt solve it you may need a new gas valve.