How did you "turn the heat on" the vent is closed on that and doubtful it's on it's own thermostat.

It's hot water not a CNG thing, they are only if the gas is out so say nothing gas works. Call a HVAC guy they would worst case tell you to contact the gas company which luckily at that point will probably be free.

As other said simplest is an air bubble if if this up high and been off the air may have connected there, easy fix should be a bleeder at least by the zone pump, Google a quick howto.