iPhone was stolen and unable to see it on Find My

What probably happened is a common phishing scam: You get a text or email from “Apple” or “Apple Security” (or sometimes “Police”) saying your phone has been found and to click on a link to see its location. This is NEVER from Apple or from law enforcement; it is from the thief or fence to steal your Apple ID and identity. Delete any such message without responding to it. Apple never gets involved in lost or stolen devices→Avoid phishing emails, fake ‘virus‘ alerts, phony support calls, and other scams

If you had the find my iPhone feature turned on and you believe someone was able to turn the feature off they had to either already know your info or as suggested you may have unwittingly given them your info via a scam text or email. If that’s the case unfortunately there isn’t much you can do. Otherwise it’s possible the battery is dead and that’s why you can’t locate it.

Not only is there no way of locating it, all of the data in your iCloud is now in the hands of criminals, so the first step is to change your Apple ID password (assuming that it wasn’t changed by the thief), then take whatever action might be needed to protect your identity from misuse or abuse. Including canceling any credit cards you had in the Apple Wallet.