If your iPhone or iPad is lost or stolen - Apple Support
Unfortunately, you won't be able to do anything with the phone until Find My is working.
What message do you see when you try to use Find My?
"No Device Found" indicates that Find My was not enabled on the phone before it was stolen.
"Offline" can mean any of the following or combination of the following:
The battery has run down
The phone has been powered off
The phone does not have an Internet connection where it is located
iPhone thieves are also very fond of placing a stolen phone in a special type of small bag that will not allow the phone to either send or receive a signal, so that may be a factor here as well.
Unfortunately, there is nothing that you can do about this other than keep checking a few times each day to see if the status of the phone has changed.
Your data is safe as long as you had a passcode on the phone, since there is no way that anyone could ever guess your correct passcode in less than 10 tries before the phone locks out completely
Be extremely careful if you receive a message that your phone has been found. This is a scam that is trying to trick you into providing your Apple ID and Password so the thieves can get into the phone.