See your doctor. I assume from your question you had a negative pregnancy test. The menstrual cycle can vary by a few days from month to month, so if your period is only a few days late, i wouldn't worry. However, if your period is delayed by more than just a couple days you should repeat the test and call or visit your doctor.
Missed 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 (i.e. PCOS). #1 reason is pregnancy.
Missed period. If you missed a period and your pregnancy test came back negative, then you most likely had an anovulatory cycle, meaning your ovaries didn't release an egg. This happens from time to time but if consistent, see your doctor because you may have a hormonal imbalance. Consider a birth control consultation to regulate your periods and avoid unplanned pregnancy.