Stolen Iphone

Find the serial number or IMEI on your iPhone, iPad, or iPod touch - Apple Support (see If you don't have your device)

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

I'm not entirely sure on that, but if you haven't already, you can log onto find my iPhone through a browser and put the phone into lost mode, so that no one is able to get into it until you disable lost mode.

If you log in to 'AppleID Central' at https://appleid.apple.com with the account the phone is registered to, you will find the list of all your devices. Clicking on the device will open a popup with all important info such as the IMEI and the S/N. Do NOT click on Remove from Account, else you will lose the ability to track it, block it, etc.