What happened when my MTN SIM is showing no signal and not working on my phone what should I do

My MTN stopped working and I couldn't received any messages, and I'm out outside Nigeria

My sim suddenly stopped showing on my phone. It reappeared later for about a day before stopping completely I have put the sim in another phone and it works what do I do

My name is ODIH Abraham Smith, am currently in Mali, my mobile phone number is 09034790242, I tried to use this Sim card this morning, and it showing me Sim card registration...