Basically you have secondary amenorrhea or absence of periods/menstruation due to some thing. The first thing to check for is pregnancy, but since you are not sexually active, this possibility is ruled out. Then other causes will have to be looked into. Stress and anxiety is a major factor. Thyroid problems and abnormal levels of FSH, LH, prolactin, androgen hormones or male hormones and estrogen hormones too can be the cause. MRI of brain may be required to rule out pituitary tumors secreting prolactin. Another major contributor is polycystic ovarian syndrome (PCOS). This can be diagnosed by a CT scan or abdominal ultrasound and tests for insulin resistance. Smoking is also often associated with secondary amenorrhea and can be worse in presence of stress. Sometimes tumors of the ovaries and brain may also cause absence of menstruation. Severe anemia and malnutrition, excess weight gain or weight loss, hypothyroidism too can contribute. If you have been on pills or depot shots then this can be post pill amenorrhea. Do discuss this with your doctor and get yourself examined. Take care!
You should ask your parents if there are any ovarian problems in your family. See a doc. But birth conrol gets your cycle on track.