It could also be you check valve at the outlet of the tank is stuck shut. This prevents water flow from coming in to the HWT via the outlet line when the tank is isolated but it allows water out of the HWT through the outlet line.
It's brass and only just around 3/4 of an inch long. Try rapping it with something to see if it will release before you drain down the system and change it if you have to.
Some HWT's have isolation valves on the inlet, outlet, and bypass, and others just have inlet, bypass, and use a check valve on the outlet.
mike