My iMessage froze up on my iPod 4th generation So I was texting one of my friends and I sent him about 700 emojis characters and now whenever I try to open the app, it just freezes for 15 seconds and then crashes out. What can I do to make iMessage work again?
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.