Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
assuming you made sure that it wasn't locked to portrait, try to force kill the app (double tap home, scroll to messages, hold it until you can close it out) then retry it. If that doesn't work, maybe a quick restart of the device.. if still no good, i would start to consider a possible restore.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.