I have zoned heating and all the other zones are working except one thermostat the thermostat show on etc but there is no heat from the base boards... I have replaced the batteries in the thermostat as the battery light was blinking what else could this be ?

If this is a hot water system the valve is probably defective or the 24 volt power is missing the zone valve. If you have a electric meter you can test for the voltage at the valve. A bad water pump can also cause this also. NOTE the valve also has what is called a end switch. If that switch is defective the furnace/boiler I think in your case will not come on when that zone calls for heat.