The answer to 'why is my water heater freezing up' is that your water heater is likely experiencing a problem with the thermostat, heating element, or insulation. This can cause the water inside the tank to freeze, leading to a frozen water heater. Some common symptoms include no hot water, water leaks, or the tank feeling very cold to the touch.
In many cases, a frozen water heater can be fixed by a homeowner, but it's important to address the underlying issue to prevent the problem from recurring. You may need to adjust the thermostat, replace a faulty heating element, or improve the insulation around the tank. However, if the issue is more complex, it's best to call a professional HVAC technician to diagnose and repair the problem.