Solution 1: Dail *#06#. if imei number missing, then Baseband mostly the problem
Your iPhone are searching or no service?
Check with another sim card. Still same problem here are another solutions:
Solution 2: Force Reboot your iPhone
Solution 3: Reset All setting
Solution 4: Remove the Sim, check for water damage and reinsert it
Solution 5: Restore the iPhone to its factory settings.