What Causes Missed Periods While On Cerazette?

I have been on cerazette for approximate 2 years and came off it last week, have read lots of disturbing results when I looked to see how long it takes for periods to return. I haven't has a period at all whilst on this pill and would like to get pregnant. I already have 2 children and had no
Problems conceiving in the past. How long do you think before periods should return? And is there anything I can do to help get pregnant?