Pumps can fail, and electrical components can fail (controls, thermostats, etc.). Additionally, excessive air may enter the system, as may low water pressure, both of which can definitely result in the system not moving the water, preventing the boiler from radiating heat correctly or at all. Water can freeze in extreme cold conditions in rare cases. Finally, if you’re using a setback thermostat, make sure it’s set to 55 degrees or higher.