The sensor that tells the boiler to turn on may be bad or possibly the leads came loose either by getting knocked or just multiple heating/cooling cycles if they weren't tightened down properly.
On most, you can test the sensor with a multimeter on ohms scale. The specs will say what the resistance across the sensor should be at any temperature. The resistance changes as the temperature changes. If the sensor is working, then the control in the boiler may be shot. If the sensor is not working, replacement is normally easy.
A good HVAC boiler guy should be able to diagnose this, but you may want to try yourself. Don't do resistance checks with power on, and you really need to actually disconnect the leads to get an accurate reading.