I have a two zonew hot water, gas fired heating system. The downstairs zone heats up. The Upper zone does not, but did just days ago. I checked the thermostat and the batterries are fine and it lights up and changes the temprature, but no effect. Either upward or downward.

If your thermostat is good, then most likely the zone valve has gone bad. When the thermostat triggers, it energizes the zone valve. This causes the zone valve to open. When the zone valve opens, it triggers another switch internal to the zone valve, which causes your furnace to fire up. The zone valves are usually located near the furnace. There should be 2 of them in your case. Have a helper cycle your good thermostat while you watch and listen to the valves. Then try the bad thermostat. If one of your valves is bad, it's probably the motor assembly on the valve. The motor assembly is user replaceable without replacing the entire valve. There is no need to drain the boiler and replace the entire valve. Good Luck

If your thermostat is good, then most likely the zone valve has gone bad. When the thermostat triggers, it energizes the zone valve. This causes the zone valve to open. When the zone valve opens, it triggers another switch internal to the zone valve, which causes your furnace to fire up. The zone valves are usually located near the furnace. There should be 2 of them in your case. Have a helper cycle your good thermostat while you watch and listen to the valves. Then try the bad thermostat. If one of your valves is bad, it's probably the motor assembly on the valve. The motor assembly is user replaceable without replacing the entire valve. There is no need to drain the boiler and replace the entire valve. Good Luck

In the mean time you can open it manually and the top floor with operate with the bottom t-stat