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
- There is a case for NOT doing step #7 where you remove the device from your account. Removing it would disable Activation Lock and make the device usable to a thief or a finder. As long as you set Lost Mode and/or erase the device, it will be not be recognized as a trusted device. This information is recommended by Apple in this thread I lost my iPhone - Apple Community
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