The answer to 'why is my water heater freezing up' is that a frozen water heater is usually caused by a problem with the thermostat or heating element. This can happen when the water heater is exposed to very cold temperatures, causing the water inside to freeze. Common symptoms include no hot water, water leaking, or the tank feeling very cold to the touch.
In many cases, you can fix a frozen water heater yourself by thawing it out and adjusting the thermostat. However, if the problem persists or you're not comfortable working on the water heater, it's best to call a professional HVAC technician to diagnose and repair the issue.