thermostat: check for proper 24 volts at the upstairs thermostat. if
the upstairs thermostat is broken you might alligator jumper its wire
pair that calls for heat to test it while the downstairs thermostat is
turned down.

water: if radiators are dry you won't get heat. check for missing water
in the upstairs loop. [let the air out of the bleeder valves while
adding water to the system.] this is normally done in summer. cold
water added to a hot boiler slowly never to a dry hot boiler or damage
can occur.