Without more info it is hard to tell . Check your theromostat to see if it needs new batteries (if it is the type that requires batteries). Do you have zone circulators or zone valves? Are the contacts in the thermostat closed. You can manually jumper the wires in the thermostat and see if this solves your problem. If you have zone valves it is very likely your solenoid on the zone valve is burnt out. Most zone valves have a manual actuation lever on them, you can try and actuate the valve and see if it allows for water flow.If you have zone circulators, is the circulator for the zone in question running? Another possibility is the circulator relay could have failed as well but that is a fairly uncommon failure. Some pictures of your setup would help greatly.