You're not ovulating. There can be causes but sometimes it just happens for no reason. When it happened to me it was just for no reason. My doctor couldn't find anything wrong. Get checked anyway to make sure your hormones and thyroid are normal because those are the two most common causes.
You can take medication to jump-start your period to keep hormones from building up or you can take birth control to regulate it if it's a recurring problem.
If there is a lot of stress it can delay periods. This could also be due to a sudden weight loss but that would have to be about 20 or more pounds. Also if you are very active with sports or running, a lot of athletes do loose their periods.
If you are really worried, I'd go talk to your doctor. They would be able to do more testing and look for any other issues.