The answer to 'why is my backflow preventer frozen' is that the backflow preventer has likely frozen due to extremely cold temperatures. Backflow preventers are installed outside the home and are exposed to the elements, making them vulnerable to freezing during winter weather.
Common causes of a frozen backflow preventer include lack of insulation, insufficient heating, or the device being located in an area with poor air circulation. Signs of a frozen backflow preventer include no water flow, visible ice buildup, or the device being difficult to turn. To resolve this, you'll need to thaw the frozen backflow preventer, which may require the help of a professional plumber if the issue is severe.