Amenorrhea: Absence of menstruation.

Causes:
Physiological (Normal) : in pregnancy, lactation, before puberty, after menopause.

Pathological(Abnormal):

Primary: Menarche doesn’t appear till 18 years.

Congenital obstructive lesions at lower genital tract.
Congenital absence of uterus, genital canal.
Sex chromosome anamoly
Polycystic ovarian disease
Hypothyroidism
Pituitary tumors
Psychogenic as in anorexia nervosa.
Uterine TB
Drugs: Anabolic steroids, drug addiction

Secondary amenorrhea: Cessation of cycles for more then three cycles.
Obstructive lesions in lower genital tract
Uterine injuries due to TB, hysterectomy, radiation
Ovarian cysts
Polycystic ovarian syndrome
Severe Anemia