It could be the gas valve solenoid is open. Remove one of the thermostat wires from the gas valve (power is off) and check for continuity between the 2 terminals. If you have an open circuit (no continuity) then the solenoid is bad.If you can tap on the valve and make it come on the the gas operator innards are bad. Gas valve replacement is imminent. If that is good I would look for a resetable clixon nearby and check it for continuity. If it turned out to be that I would push the button and reset the safety, brush off the burners, vacuum out the heater and check the smoke stack for an obstruction.