Phil,
Check the electronic controller in the Aqua Hot bay. First thing I'd so is to push the reset button with a non-metal probe and see it that resolves the problem. If not, verify that both circulation pumps are working, the zone indicators are illuminated and the low voltage cutoff is not red. Indicators for all these should be green. If any are red, you may have an electrical overload condition in that circuit. You may have 4 zones, Living Kitchen, Bathroom, Bedroom, and Tank bay.
There is a downloadable shop manual with a trouble shooting guide on the Aqua Hot web site also.
Steve