What to do if my iPhone was stolen?

The ONLY way someone else could have activated your phone would have been for you to NOT have enabled Find My on the phone nor use a passcode to open the phone. If you didn't have either, your only real option is to have the phone Blacklisted by your cellular provider.

See this link --> If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

jhamndz wrote:

Passcode can easily be removed if you restore an iphone.

NOT without the Apple ID and Password used to set up the phone it can't. Even the FBI could not bypass an activation locked iPhone. If the phone wasn't activation locked, it's like leaving your car keys in an unlocked car and wondering why your car was able to be started and driven away.

How did they get passed the Face ID and passcode?

Passcode can easily be removed if you restore an iphone.

There is a slim chance of you getting it back. If your find my iphone is on, you can track it down if the phone and cellular data is on. You can report it was stolen tho, all you need is your imei