The phone will show up in Find My if the battery is still good.....and.....the phone has a connection to the Internet.
If the phone does not show up currently, either the battery has run down or the phone does not have a connection to the Internet where it is located. Or, both of these things havenoccurred. Sometimes....but not always.....a "last known location" will be displayed for a day or two in Find My.
There is really nothing that you can do at this time other than checking a few times each day to see if the status of the phone has changed.
iPhone thieves are very fond of putting stolen iPhones in a special type of bag that prevents any kind of signal from reaching the phone, so the phone can't be tracked by the owner. That might be in play here as well.
"Apple" or "Apple Service" will never send you a message about the phone, so be careful with scams.
If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support