Hi Vandana,
Unhealthy way to loose weight through crash diets will lead to lack of proper nutrients from our body which can even drain off our muscles mass which will lead to weak muscles and decrease in immunity power of body. Loosing weight in a healthier way may help to maintain your fitness level and even it increases as it includes exercises and healthy dieting. Try resistance training exercises like crunches, reverse crunches, Heel touch, lunges, Squats, etc. and cardio activities like running ,jogging, Brisk walking ,etc. for total of 45 min which can help you to achieve your goal in healthier way.