My iPhone has been stolen

Sorry to hear that you have lost your iPhone.

To locate your iPhone in the Find My [Device] app, you must have activated the Find My app BEFOREthe iPhone was lost. Here is some information on that app ➔ If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support.

Note that your lost iPhone must be able to make the connection to the internet via Wi-Fi or cellular connection so that it can mark its location and broadcast it. 

Axel F.

Sorry to hear:

Go thru these steps:

https://support.apple.com/en-gb/HT201472