The answer to 'why is my supply line frozen' is that the water supply line leading to your home or appliance has likely frozen due to cold temperatures. This can happen when the line is exposed to freezing air or not properly insulated.
Common causes of frozen supply lines include inadequate insulation, a drafty area where the line runs, or a sudden drop in temperature. Symptoms may include reduced or no water flow, visible ice buildup on the line, or the line feeling very cold to the touch. Solutions can include insulating the line, using a space heater to thaw it, or calling a professional plumber if the issue is severe or you're unsure how to safely address it.