Hi Adamu, you need to contact MTN support to know the type of barring on your line, there could be many causes for that. It could be SIM registration barring or line termination issue due to inactivity.
You can contact MTN Nigeria support via any of these channels:
1. MyMTN App
2. Facebook: @MTNLoaded
3. Instagram: @MTNNG
4. Live Chat: http://bit.ly/2OC5bHz
5. Email: customercareng@mtn.com
6 .WhatsApp: 09033000001