What Causes Missed Periods While On Microgynon?

i have been on microgion for 3 years now and its been great no problems, but 3 months ago i started to be on my piriod i thought it would be fine but then it had been 3 months and it still had not gone i went to the doctors and they gave me another type of pill i tryed that but its been a month now and it still hasnt gone what should i do please help.