If the app store shows "open" and not "get" or "download", then the app is still on your phone, you just have to find where the icon is hiding. You may have accidentally rearranged or moved things.
Swipe left all the way to the end of your pages, then swipe once more and it will bring up your App Library. Scroll through the app library until you find the missing app, then tap-hold the icon and you'll see an option to add it back to your home screen.
If that doesn't work you can always delete the app then reinstall it.
