Was the head working before or after you got out on the water? The 120V system (House System) uses breakers for Refrigerator, outlets, hot water heater, Air Conditioning, battery charger, electric stove. The 12v system which runs off of the batteries uses fuses for bilge pumps, water pump, 12 side of the refrigerator, electric head, lights and starter). Most of the 12v fuses are underneath the dash. Some, like the fridge have dedicated fuses on the device itself.
Did you try switching the battery selector from one battery to the other. Maybe your starter battery is isolated from the house. Maybe one of hte batteries was dead. If that doesn't work, I'd take a voltmeter to the switch in the head and the head itself and see if you're getting 12v there.
When exactly was the last time it worked and were you on shore power or battery at the time?