My iMessage froze up on my iPod 4th generation
You can double tap the home button till the app switcher comes up at the bottom of the screen, then press and hold the messages app until the little red minus button appears at the top left of the app button. Press that to shut it down, then open it again. 

You could also hard reset the iPod by holding down the home and sleep button at the same time for about 15 seconds.