The answer to 'why is my P-trap frozen' is that the P-trap, the U-shaped pipe under your sink, has likely frozen due to cold temperatures. This can happen when the water in the trap freezes, causing a blockage and preventing water from draining properly.
Some common causes of a frozen P-trap include: cold air drafts, uninsulated pipes, or a lack of heat in the area around the sink. Symptoms include slow or no drainage, gurgling sounds, and water backing up in the sink. To fix a frozen P-trap, you'll need to thaw it out, which may involve using a heat lamp, hair dryer, or other heat source to warm the pipe. If the freeze has caused damage to the pipe, you may need to replace the P-trap. In some cases, insulating the pipes or adding heat tape can prevent future freezing.