Causes of secondary amenorrhea include stress and anxiety, malnutrition, depression, overexercising, sudden weight
loss or
gain (obesity),thyroid disorders
and hormonal imbalance due to polycystic ovarian syndrome(PCOS).Smoking is also associated with secondary amenorrhea. Sometimes tumors of the ovaries and brain may also cause absence of menstruation.
You should consult gynecologist for internal examination and other tests needed to detect it's cause.