My iPhone was stolen and I can't find it

Assuming that you enabled Find My on the phone, the phone must be powered on and have a connection to the Internet for the Find My feature to work.

If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

In addition, the chances of you ever finding it are almost zero.

If you used a physical SIM get your provider to 

Cancel your SIM.

Send you a replacement SIM on same number.

You can then put the SIM in another iPhone to get the codes you will need.

Phishing warning

Its possible for a thief to obtain your phone number particularly if you did not have a PIN set on the SIM.  So do not respond to texts to sign into your account! They are NOT from Apple. 

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

The Complete Guide:

▷ Follow these important steps.

▷ Call 1-800-MY-APPLE.

▷ Learn how to file a claim.

▷ Get Support.

▷ Download the Apple Support app.