Thank you for replying back.
As you are 17 year old now & your periods started at age of 11 years.So by this time you should have settled with your menstrual cycle & should have a normal periods now.
As you do not have the periods since last 9 months,this is not serious but is of concern.
The most common causes of missed period are:stress,being overweight or underweight,imbalance of hormones,polycystic ovarian syndrome/estrogen dominance,uterine abnormalities (fibroids/cysts/polyps/endometriosis).
You need clinical examination,testing for thyroid function and other hormone levels. A pelvic exam is required to rule out any uterine abnormality, a cervical polyp or fibroid, or a uterine infection.Also you need an ultrasound to evaluate the uterus, the ovaries and the fallopian tubes. So I advice you to consult a gynecologist for the clinical examination & tests to find out the root cause & then the treatment will be done accordingly.
If you still have any questions,please let me know..I am happy to address your follow-up questions.
God bless you & take care.