Phone stolen

This support article has a list of things to do if your iPhone, iPad, or iPod touch is lost or stolen. --> If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

If Find My iPhone is offline or not working, read this Apple Support article to get help; e.g., when you don't see your device listed, or you see an unexpected device or an alert that says your device is offline. --> If Find My is offline or not working - Apple Support

Assuming that you enabled Find My on the phone......before.....it was lost, the phone will display as "Offline" using Find My if the battery has run down or the phone does not have a connection to the Internet where it is currently located.

In that case, all that you can do is keep checking a few times each day to see if the status of the phone has changed.

Unfortunately, iPhone thieves are fond of placing phones in a special type of bag that will not allow the phone to send or receive any signals, so that might be a factor to consider here as well.