Does muscle turn into fat when you stop lifting weights?

No, muscle and fat are two completely different substances which cannot be interchanged. Weight training causes muscles to hypertrophy (increase muscle mass) and burns calories both from the exercise itself and the increase in muscle mass. When weight training is stopped, muscles atrophy (lose muscle mass). The calories expended from the weight training exercises and the increased muscle mass are no longer a factor, and therefore the body's caloric requirement is reduced. Body fat is subsequently gained when the caloric consumption remains the same. Basically, body fat is gained because the calories consumed exceed the calories expended.