How late can your period be if youre underweight ? & when will it come once i gain the lost weight?

Anorexia?: This is much too complex an issue for an answer on a forum like this. Low body weight, often with good aerobic conditioning, can delay menstruation or stop it entirely. Without lots more information, including physical exam and maybe blood tests, there's no way to know if weight gain will help. You need to be in a doctor's care to evaluate this situation and recommend solutions. Good luck.