My friend Larry Weingarten will likely have something to add to this, but the first thing that pops into my mind is that the FVIR mechanism might have been triggered. That brand usually has an FVIR reset button. You probably no longer have the documentation, but ought to be able to find out where the button is from the maker. Also, you should have someone replace the sacrificial anode. The heater is old enough for that to be used up. If it’s gone, the heater will rust out sometime soon.