My iPhone was stolen and suddenly disappeared from Find My

Was the thief someone you know? Did the thief have easy access to your Apple ID and/or iPhone passcode?

See this article.

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

If the battery has run down or the phone does not have an Internet connection at its current location, the phone will not show up on the map. The phone may also have been powered off.