Here is a shot in the dark. Have you tried doing a Spotlight Search for one of your contacts or for something that you know that you discussed in an email with someone? If the search process any results, (and it should), tap on one of those emails and see if the mail app launches. If it launches, maybe the icon will come back.
You probably already know, but swipe down from the middle of any home screen to get to the search feature (or swipe to the right from the first home screen).
Another shot in the dark and this is a long shot. Did you use restrictions? If you restricted Allowing changes to accounts and then you deleted the mail app, I wonder if that would cause it to be hidden. I know that you can not restrict mail like you can other apps, but I'm just trying to think of anything that would cause what you are seeing.