Hi Muqadas, it may be a temporal network issue, sometimes, if you power off and power on your device, the network will start working well; however, if the issue persist, you need to contact MTN support for resolution.
find below MTN Nigeria Contact 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
7.Call: 180