This document 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
Use the Find My app to locate a lost or stolen device - Use the Find My app to locate a missing device or item - Apple Support
If this is another person's device the person who lost the device "can also use the Find My app on a device owned by someone in [their] Family Sharing group, or sign in to iCloud.com/find to locate [their] device using a browser. [ Locate a device in Find Devices on iCloud.com - Apple Support ]"
If Find My iPhone is offline or not working click on the link to get help 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