@cween,
Hi!
Missed or delayed periods is not a rare case. First of all make sure you are not pregnant. As about the other causes of the delay they can be very different. For example: bad diet, stress, excessive physical activity, hormonal imbalance or some health disorders (e.g. thyroid problems). By the way you haven't indicate your age. If you are a young girl and your periods have started just a few months ago, then do not worry it is a normal condition related to the hormonal system that requires some time for regulation.