What Is The Treatment For Amenorrhea?

I currently have amenorrhea. I am 18 years old, almost 169cm , and around 48 kg. Once, I reached a weight from 50 kg to 45 kg , I lost 5 kg for almost 5 months , not always with healthy methods and my calorie intake was low. After 4 months of being 45 kg I increased my calorie intake, but I dont have a mestrual cycle anymore .. my menstrual cycle wasnt regular since january 2010 ,, they come just once in 5-6 months. what should I use to get my menstrual in return.