Some medications may cause irregular or absent periods as a side effect. Polycystic ovarian syndrome and hypothyroidism may also cause irregular or absent periods. Low body weight is another possible cause. You should see a doctor to rule out serious illnesses with this effect.
There are many causes of amenorrhea, which is the medical word for absent or missing periods. Mental stress is one cause. Very low body weight or a pattern of extremely intensive physical workouts can also cause your menses to stop. Hormonal imbalances or thyroid problems could also be the culprit.
Stress, poor diet (overeating, under-eating, or nutritionally empty diet), hormonal imbalance, smoking, caffeine, excessive alcohol consumption, and a whole range of medical conditions and medications can mess up your cycle.
Ovarian cysts can do weird things to your cycle. If they are big enough they often get in the way of your period and can even stop it all together. When they are this big they need to be removed, nothing else can be done about them. Some STD's can change your period. Talk to an OBGYN.