Why is my Sim ging off network for hours then network will be restored and this have been going on for two days now

I was using my phone some days ago on the internet though and suddenly the network was gone ad I turned my phone off and on removed the Sim card and put it another phone but still not working but other mtn sim cards in my phone are working so what is the issues I really need it to be fixed because I have don my nin on this sim and all my Sims but only this one is having issues.