Apps can be removed from the home screen without being deleted. Go to the App Library (swipe left past your last home screen). From there you can touch and hold an app and slide it to the home screen.

Also note the Search window at the top of the App Library.

If the Find My app is not anywhere on your phone, it may have been deleted. To reinstall, see the following from Delete built-in Apple apps from your iPhone, iPad, or Apple Watch - Apple Support

Restore a built-in app that you deleted

On your iPhone, iPad, or Apple Watch, go to the App Store.

Search for the app. Make sure that you use the exact name of the app.

Tap the Redownload button 

Wait for the app to restore, then open it from your Home Screen.

Other information about finding lost devices:

Use any internet browser and log into

Also see the following Apple articles for information about lost or stolen iPhones or iPads: If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support and

Use the Find My app to locate a missing device or item - Apple Support