HOW TO FIND MY IPHONE X

What to do if your phone is lost or stolen.

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

Assuming that you enabled Find My on the phone before it was lost.......

Open Safari

Type in www.icloud.com/find

Enter your Apple ID and Password

Select the iPhone from Devices

If the battery has run down or the phone does not have an Internet connection, it will likely be displayed as "Offline". Nothing that you can do other than keep trying.