It is probably not this easy, but the first place to check would be the multitasking bar. Double tap the home button and this bar will come up. Scroll to the left, and you will see iPod controls along with another icon with an arrow on the left side. This last icon locks the phone into portrait orientation. If there is a padlock inside this icon that means it is locked in portrait, just touch the icon, the lock should go away, and your screen should flip as you wish.