Find my iPhone

There is a "Find My iPhone" and "Find My Friends" app that come preinstalled on iOS devices. I don't know anything about a "Find My Family" app. If either of those apps is truly gone, you should be able to re-download them from the App Store:

Remove built-in Apple apps from the Home screen on your iOS 10 device or Apple Watch - Apple Support

But first thing I would try is to search your iPhone and see if it might be "hidden" (in a folder) somewhere:

Use Search on your iPhone, iPad, or iPod touch - Apple Support

That would be interesting to see. Seems that according to this support document, this is how you locate members of your Family Sharing group, through Find My Friends app, and then help them locate their devices with Find My iPhone. Share your location with your family - Apple Support

I don't use Family Sharing with my iPhone, but I've not noticed any differences in the operation of Find My iPhone. It seems to be working just fine as far as it goes with my iPhone, iPad, and Apple Watch. I know that my son uses it with his phone to track his wife's phone. I can check with him later when he is finished at work to see if he has seen any problems. They both have the iPhone X, and he is on iOS 11.2.6.

Okay, so just to make sure we are talking about the same thing here. Your subject is Find My iPhone, but later you mention Find My Family (which I missed until you mentioned Family and Friends together in your last response to me), and then say that Friends appear OK, but Family is gone.

Find my iPhone is designed for devices signed in with your Apple ID, but if you are using Family Sharing, you can also track your other family members if they are members of your Family Sharing, and you are sharing their location.

Find My Friends is a different app, which you can find the location of, and share your location with friends that have accepted an invitation from you to join Find My Friends. Let's make sure we are speaking about the correct app and the correct following here.

FYI, my son responded to me about Find My iPhone with his Family Sharing group, and all is working correctly for him.

I've never heard of an Apple app called Find My Family. It could be something you got from the App Store, or something from your carrier? But is isn't anything that any of us have been talking about here it seems.

Can you rephrase the question so we can figure out what specifically you want to know?