The answer to 'why is my pressure regulator frozen' is that the pressure regulator can freeze up in very cold weather, usually below freezing temperatures. This is a common issue that can happen when the water inside the regulator freezes, preventing it from functioning properly.
Some common symptoms of a frozen pressure regulator include low water pressure, water not flowing, or the regulator making strange noises. To fix this, you'll need to thaw out the regulator. This may involve wrapping it in a heating pad or towel, or using a hair dryer to gently warm it up. If the regulator is severely frozen, it's best to call a professional plumber to inspect and repair it to avoid any further damage.