HT3529 Messages app icon gone from main screen

The green "Messages" app is gone from my phone's main screen.  I scrolled through all my apps and it is not an any screen.  If I hit the bottom button on my phone twice, the app shows up in the bottom row with other open apps, but I can't get it to go back onto my main screen. How do I get it back where it belongs?

Thanks!  ~ Kim