FaceTime app was on my iPhone and now it is gone. Not sure how it happened. How do I get back?

There is no Facetime app; never has been. Facetime is built in to iOS; to use it select a contact and tap the Facetime button.

What Facetime app are your referring to? In Settings?

There is no app on the iPhone to use Facetime you do it through your contacts or the Phone app.

FaceTime is accessed from within the Contacts App. There is no stand-alone App for FaceTime.