Several things could be wrong. A likely cause is a change in diet and exercise- have you lost or gained a lot of weight recently or started a new training regime? Sudden changes from your body's "norm" can cause a disturbance in a menstrual cycle.
Do you have any other symptoms, though? Any funny feelings of pressure or pain? If so, go see a doctor- you might have a cyst. If you feel like something more is wrong, then definitely get it checked out for sure- there are many causes for a skipped period, and while it's most likely to be something harmless, if you're worried, you should get checked just in case.