Delayed period. Reasons for missed/late periods: low body weight, obesity, marked weight ^ or v, over-exercise, endurance athletics, breast-feeding, ^ stress, illness, eating disorders (bulimia or anorexia), anovulatory cycle (no egg released), travel, hormone irregularities, drug use, meds (i.e., birth control) or medical problems. #1 reason is pregnancy.
So many causes. There are so many possibilities. The most common is stress. Periods are regulated by an interaction of the brain, the ovaries and the uterus. If your brain feels it's not a good time to get pregnant (due to stress for instance), the correct hormones might not get released and you might be late. Other causes are low body fat, polycystic ovarian syndrome or other hormonal imbalances, tumors, etc.