A number of conditions including chronic illness, medications (Depo-Provera or DPMA, starting chemotherapy, Transgender patients taking testosterone), severe weight loss as seen in anorexia nervosa, polycystic ovarian syndrome, menopause - all can cause episodes of amenorrhea (lack of menses). Additionally, irregular menses are typical for a young woman during the first year of her menstruation as her hypothalamic-gonadal axis is maturing.
Amenorrhea is also categorized as primary (no menses yet by age 16), or secondary (cessation of previous menses for 3-4 months).
More information would be needed to determine why a particular woman is experiencing secondary amenorrhea.
If you or someone you know is experiencing menstrual irregularities, you should consider being evaluated by your health provider or women's health specialist.
To learn more about amenorrhea, check out this link at WebMD:
http://www.webmd.com/infertility-and-reproduction/guide/absence-periods