No service problem

I have been using iphone for a quite long time but never faced problems like this. I have an iphone 7, after updating to the ios 12.3.1 a few days later my iphone start saying no service, i restarted my iphone network started to appear for sometime and the disappeared. After that it has been three days and my phone is showing no service. I am unable to receive calls, messages etc and i am very much disappointed with this phone. Please someone help.

PS: English is not my mother language. Sorry for the mistakes.