All radiators but one are toasty. The cold one is up on the second floor, and is the last one on the line, farthest from the boiler.
It has been bled of air. It shoots out a thin stream of (cold) water in a small arc when the bleed valve is opened.
There is a supply main and a return main (black iron pipe from the 1940s).
Boiler is cutting off as expected at about 188F, with pressure reaching ~21psi.
The expansion tank is empty. It rings hollow. (I know from firsthand experience the dull sound of one whose bladder has failed so that the tank is full.)
This radiator was working fine until we lost electrical power a couple of weeks ago after a storm, and so the boiler was non-functioning for 24 hours. I thought perhaps the line had become frozen, but wouldn't the small arc of water from the valve indicate that there is no blockage?
EDIT: On the cold radiator in the new room, there is a shutoff valve on both the return and the supply. There's an arc of water from the bleed vent when supply is open and return is shut, and when the supply is closed and return is open (and of course when they're both open). But the water is cold even when the boiler is firing and the circulating pump is on and the system temp has reached 185F and about 22psi.
Best Answer
The water that is being vented or bled out when you open the vent could be coming up either the supply or the return line. You could freeze one line and not the other. Is the circulating pump for the boiler running?