The answer to 'why is my sewer line frozen' is that sewer lines can freeze when the temperature drops below freezing for an extended period of time. This can happen if the sewer line is not buried deep enough underground or if there are cracks or gaps that allow cold air to get in. Symptoms of a frozen sewer line include slow draining sinks, toilets that won't flush, and gurgling sounds coming from the plumbing.
The best solution is to call a professional plumber who can use specialized equipment to thaw the frozen line safely and efficiently. Attempting to thaw the line yourself with a space heater or other DIY methods can be dangerous and may even cause further damage. A professional plumber will have the right tools and expertise to get your sewer line flowing again without any risk to your home or personal safety.