Muscle tissue is highly active. Every pound burns about 35 calories per day, just to maintain itself. This "maintenance" process is a big part of your natural metabolism, or calorie-burning system. When you lose muscle your metabolism slows down (often permanently!) and it becomes harder and harder to lose weight and keep it off.
So what's the solution? Begin a basic strength training program which involves working all the major muscles in the body at least once or twice per week. The easiest way to add muscle is with weights and weight machines but you can also use your own bodyweight, rubber fitness bands, etc.
If you're a beginner make sure to consult with a personal trainer who can teach you proper strength training/weight lifting technique before you begin. Also, most people only need to perform strength training about 3 times per week, 40 minutes per session, in order to see good results quickly.