What has stopped my combi boiler working suddenly?

My Saunier Duval combi boiler (sorry no model no.) has suddenly stopped working. Before, a green light would come on when the boiler was switched on - now the green light flashes constantly but there is no hot water. Is this likely to be an electrical fault or a pipe / valve issue? Sorry I can't give more details but I came back from holiday and found it had stopped working. Any comments please - would like educated guesses as to the cause. Boiler is not that old and can't afford a new one! Thanks.