What is the pathophysiology of amennorhea?

Amenorrhea is caused by pregnancy, delays in ovulation, menopause and polycystic ovarian syndrome, as well as ovarian failure from infection, etc.; the "patho-physiology" occurs when the normal fluctuation of the estrogen/progesterone cycle is absent, and withdrawal bleeding due to degeneration of the uterine lining does not occur.

Great Wikipedia question. Very complex. You might want to review you health class notes as well.Or spent $20 and read uptodate.com for a week which will give you all the details you want/need to grasp the complexity. Good researching!