The answer to 'why is my boiler freezing up' is that a frozen boiler is often caused by low water pressure, a malfunctioning thermostat, or a lack of insulation. Symptoms include the boiler not heating up, water leaks, and the system shutting down. Solutions can include bleeding the radiators, adjusting the thermostat, or adding more insulation around the boiler and pipes.
If the freezing issue persists or you're unsure of the cause, it's best to call a professional HVAC technician to diagnose and repair the problem. Attempting DIY repairs on a boiler system can be dangerous if you're not properly trained.