Like OTH said, make sure there is a good flow of air into and out of the flue.
First port of call...make sure there's no obstructions... plastics bags, bird's nests, wasps' nest, cobwebs, etc.
My combi stopped working because of cobwebs in the venturi (a thin pipe that detects pressure differences). Once it was cleared, it started working again.
Anyway, glad you got it sorted...Wrong sort of weather to be without hot water/heating.