It happens that one way or the other your Sim card got lost or stolen. In my own case which is my personal experience my phone got missing in a keke and all efforts to reach out to the driver failed.
So I had to choice than to retrieve the MTN sim, first thing is to make sure you are the person that registered the Sim card.
After then you need to visit the MTN office near you, I did mine in Bodija, Ibadan in oyo state. And I was charged about 700 Naira, took about 4 hours to start working.
Hope this will help someone who might have lost him/her sim card.