Hello,
Welcome to ehealthforum and thanks for writing
If you had unprotected intercourse in the past then amenorrhoea(no periods)can be due to pregnancy.You need to do a Urine pregnancy test for this by a fresh morning urine sample.Some other causes can be hormonal imbalance,underlying causes as ovarian cysts(commom in young woman)fibroids,endometritis,thyroid hormone disorder etc.
It requires a detailed examination and investigations as USG scan and Hormonal analysis which would help in ruling out the cause.Kindly visit a Gynaecologist for evaluation.If hormonal imbalance is detected then you might be prescribed oral pills for inducing periods.
Take care!