We had a similar problem a couple of weeks ago.But ours was the heating not the hot water.Do you have a radio frequency thermostat? Ours had temporarily lost the signal and the boiler had switched itself off. Sorry if this is way off the mark but it was not in the manual and the technical line at Worcester Bosch explained to me that our wireless lap top or portable phone had caused this.Shortly after it reset itself.A new phenomonon (sp?) apparently. Hope you get it sorted.
Thanks but we have no wireless equipment, I am currently at work so I'm receiving fraught calls from home, I'll have to fish out the handbook later. Regards Sherb