From what I can remember with 8.1 the message app was tied to the OS. My question is; how did you lose it in the first place?
Did you accidentally unpin it from the startscreen? Swipe to your "all apps list" (swipe right) > tap on of the capitalized letters (Example: A or B) > this should give you an alphabetical grid on your screen > tap on the letter "M" > will take you straight to all the apps starting with "M" > check if Messaging app is present?
If the app is present > long tap and select "pin to start" > the app should be back on your homescreen.
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