Legally your boiler should be fixed by a Corgi registered person. Best contact Worcester.
There is an air flow differential switch in the flue fan circuit which could drop out (electronically speaking!) and stop the boiler running. This could be for any number of reasons - flue restriction - flue fan failure - faulty diff switch and so on.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix home heating systems.