The "Find My" feature on the iPhone uses the device's GPS, Wi-Fi, and cellular data to determine its location. If the device is offline, meaning it is not connected to Wi-Fi or cellular data, it will not be able to be located using "Find My." Also, if the device has been turned off, it will not be able to send its location information to iCloud. In this case, the last known location will not be displayed in "Find My."
It's important to note that the "Find My" feature is not foolproof and it may not always be possible to locate a lost or stolen device, especially if it's turned off or not connected to Wi-Fi or cellular data. However, if you haven't already, it's still a good idea to mark the device as lost in the "Find My" app, so that it can be locked or erased if found by someone else. Additionally, you should also contact your carrier and report the device as lost or stolen, to prevent unauthorized use of your cellular service.