No heat/hot water sucks. Glad you finally figured it out, I would have given up after all that and called someone.
I had an issue with our gas furnace a couple years ago. It would try to fire repeatedly, then shut off. I googled the problem, and it looked that it might be a flame sensor (safety thing) that ensures the gas is lit, otherwise it shuts everything down. The sensor can get a form of corrosion on it over time that keeps it from sensing there is a lit flame, so the furnace will not light. a wire brush and some sand paper, and we were back in business. If you have a gas furnace, keep that in mind, could save you an expensive service call!