Suggest Remedy For Missed Periods

I am 31 years old women. I am taking the duphastone 10 mg from last two month. In first month, period was came just after finishing the duphastone course but in second month the period did not come even duphastone course is finished . My period cycle is finished but period did not come. I have also done preg test but it is negative. What could be the reason, please explain.