What Causes Missing Periods?

Doctor,i m 20yr gal. Weight 67kg.. I didnt sex.. But since 3months i m not having my monthly periods.. Last 3day ago i went to a senior gynologist and he prescibe me to take Deviry 10mg twice a day for 5days.i took and today is my 4th day,but still i didnt got any initial of periods. I m really scare. Help me