My iPhone 3G keeps kicking me off my message app, why is this happening?

My phone has been kicking me out of my main iPhone messaging app. And I don't know why. Before this happened my friend had been texting me long emoji texts then I go to open one up and it just kicked me off and took me to the home screen. I've tried turning it off. Restarting. And updating my phone. But nothing worked. How can I fix this because I can't open up the app to reply to really important texts.

Preform a hard reset.
Hold your lock button and home button down, when the "turn off phone" appears ignore it and keep holding your buttons. When the apple logo comes on, ignore it and keep holding your buttons. Then you will see the screen change and then you can let go of the buttons.