What Causes Absent Periods After Discontinuing Contraceptive Pills?

My wife is 26 years old. We are married since Nov,2010. She has used contraceptive pills till Mar, 2011. Since the time she stopped the pills she dit not get the cycle. When we tested home for pregnency it was negative. She has not got the cycle till now. Its been 15 days since cycle due date. Pls suggest . We need children as soon as possible.