How do you restore the message app in iphone

I don't believe you can delete the "message app" from your iPhone. I is probably hidden in a folder. Search for it on your iPhone:

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

If I can add my two cents to what Javaliga and Limnos correctly suggested (you can't delete Message app, so that app is still there), if you can't find your message app icon, you can go to Settings>General>Reset>Reset Home Screen Layout.

BUT....but, before you do it, keep in mind that performing this reset all your apps icons could be moved to different screens, and you will loose all your created folders (then you have to move your icons manually, re-create folders, and so on). SO, my suggestion should work, but it can create a chaos on your already personalized screens (unfortunately iTunes program on a computer doesn't manage / arrange apps any longer).

Regards

Giulio

Are you sure you didn't just accidentally tuck it away in a folder somewhere, or lose it on a dozen pages of apps?