You are missing your periods because you have gained tremendous weight in past one year.
Diagnosing the cause for your weight gain is most important not only to reduce your weight but also to bring back your normal periods.
The most important cause for weight gain in a teen girl like is polycystic ovary syndrome
which is an hormonal
disorder where ovaries produce excess of androgen hormones and increased insulin resistance
which results in obesity,excess hair growth and missing of periods.
The other causes for weight gain in our case could be ...
Lack of sleep.
Use of antipsychotic
medications or medicines like steroids.
Use of contraceptives like injectable Depo provera.
You require a consultation with an endocrinologist
for proper evaluation and necessary investigations to find the cause.Once the cause is identified,treatment would become easier.
It is said that a mere reduction of 10% of weight results in normalising the periods.
I hope it helps.