Your condensate valve could be blocked; in the recent cold weather it might have frozen and the boiler will shut down for safety reasons. If you have an external condensate pipe try putting a hot water bottle around it for an hour or so before resetting the boiler, that should dislodge any ice in the pipe. Alternatively if your condensate pipe is indoors and runs to the main drain it could be blocked with food/soap scum from the taps/washing machine. Either call an engineer or do as I did, open up the condensate pipe and blow down it until the blockage plops out the other end!
NMn
March 2010