The App Store app on my iphone 6 has disappeared. How do I get it back?

You cannot delete it, so it has probably been moved somewhere you can't find it, either another Home screen page or into a folder. You can verify it is there using Spotlight search: drag your finger down starting from the middle of the screen, and search for "app store". If it appears it is somewhere on your phone, and if it's in a folder the folder name will be shown next to it.

It's also possible that it has been blocked in Settings/General/Restrictions. If it doesn't appear in the search this is the reason you can't find it.