What Causes Irregular Periods?

I don t have periods for months. I consulted a doctor a year before she prescribed me to take NOVELON for 42 days and asked me to wait for next cycle to occur without NOVELON. But I didn t had any cycle. She suggested me to take NOVELON for next 42days I did so...but I didn t had periods after stop taking NOVELON..now it has been 6months I didn t had periods. What is the problem. And what should I do.