It would seem your heater valve is shut closed. As TX mentioned above it might be loss of vacuum at the valve or faulty check-valve. To establish this disconnect the vacuum pipe attached to the valve bell (at the nearest junction) and apply suction. If the valve opens (needs constant vacuum to stay open) it would imply loss of vacuum somewhere in your system. If applying suction does not open the valve then the diaphragm in the bell has perished. As a temporary measure you could tie the lever on the open position so you don’t freeze until you can replace the valve.
