Restart your iphone (turn it off, wait a while, and then turn it back on)
If nothing changes, read this support guide, following all the steps provided:
If you see No Service or Searching on your iPhone or iPad - Apple Support
If the issue still persists, have your iphone inspected at the nearest Apple Store or Authorized service provider.
Check here: Apple - Find Locations - Location Selector
Regards
Giulio