iPhone 7

There can be several reasons.

1. An issue with your service plan. Contact your carrier to find out if there a problem with it.
2. An issue with your carriers network or local tower where you are. Again contact your carrier to find out why.
3. A Problem with your SIM card. Contact your carrier to get it looked at or replaced.
4. A Problem with the iPhone cellular hardware. See here: link->
   1. If you see No Service or Searching on your iPhone or iPad - Apple Support
   2. iPhone 7 Repair Program for "No Service" Issues - Apple Support

faiza148 wrote:

My iPhone 7 model is not included in apples repair program,it does not seem fair

That means it's not actually suffering from that issue. Only the included models are known to suffer from that issue. If yours is not there, then the reason for the "no service" message is something else entirely and has nothing to do with the issue reported in those models.

Contact your carrier, and haver them check your SIM card and service plan as suggested in the other link provided.