It's not an app you can actually delete. But it is an app you can turn off in restrictions. So go to Settings > Restrictions and see if the message app is somehow turned off. If it is turn it back on.
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It is not possible to delete the Messages app from your iPhone. You have probably misplaced it somewhere on the Home screen. Try searching for it by swiping down from the middle of the screen and using the Search. If it is in a folder the folder name will be displayed; if not, it is on one of your Home screen pages.
Question: i accidently deleted message app from my phone and dont know how to get it back. help please!!!