What Causes Delay In Periods?

Hello Sir !
I am a girl of 21 years and married.. I had my periods date on 12th . But til today I couldn't have periods. The white fluid that ussualy comes before periods date has also stopped since 11th. I doubt that if I am pregnanant so for this I also took oral contraceptive pill of Unwanted 72 withing 72 hours of 11th night . Please suggest what can be done and I dont want an unplanned child. So will this contraceptive pill have helped me that I have already taken . Please suggest some way and explain why didn't I had my periods?