You can't take the Fisherman out of this Camper wrote:
If it has the OPD valve it (the automatic OPD valve) may have closed if you opened the hand valve too quickly. Try turning it off... wait for a about 10 seconds... if you hear a slight 'click' that will mean the OPD valve has dropped. Then try turning it on very slowly so that the pressure on both sides of the valve can equalize and try the stove/hot water tank again.
The OPD on a permanent ASME tank is part of the fill port, not the service valve as it is on removable DOT cylinders. Your advice is good, but it applies to the high flow preventer valve in either case, not to the OPD. The high flow preventer is part of the POL service feed connector.