Hello, In this age group, missing period occurs due to improper ovulation and may be associated with hormonal imbalance, emotional stress/ anxiety, weight gain/ loss, irregular diet habit, thyroid disturbance, ovarian cyst etc. One pelvic ultrasound and hormonal assay are quite helpful to rule out underlying pathology if any. She needs to improve lifestyle by taking healthy diet with more vegetables & iron supplements, avoid stress by yoga/ meditation, drink more water, maintain genital hygiene, do light exercise and take sound sleep. Hope it helps for her. Be well.