Did you drop it?
Hold top ytop button and home button together for 10 seconds then let go and see if reboot helps.
The hard reset Weth suggested should work. I've had freezing issues on my messages app. I think mine are because I use 3rd party keyboards. When it freezes up, I just swipe the messages app out of the App Switcher. Or delete the messages app from the background apps. That works.