I can't load or use my MTN line, how do I reactivate or get it working

I’m not able to use my MTN app. It’s not just working. I can’t get Otp too.

My MTN line have not really been working for some day now, the bars signifying network is always there on my phone but I can't check balance or do anything ussd. I can't call...

The message I keep getting when i try to call my Mtn line is that I can’t receive calls at the moment