What Causes Irregular Periods?

Hi! I have huge problem. I have not had menstruation for about 2 years. Is it very bad? The reason probably is my underweight and influence of anorexia. I think I have understood, that I need to eat more, but my biggest concerns are about the menstruation. If I start to eat properly, the menstruation will show up just like that or I need to take pills. Maybe it is possible that I won't have menstruation after such a long period at all?Sorry for my bad English, I'm a foreigner. Thanks