Double click the home button and swipe left to right. There is icon on the far left that looks like a circling arrow. If it had a lock symbol in it then your locked in Portrait mode. Tap it and it should unlock.
Docd, thank you thank you! I'd been all over settings repeatedly, with no luck. Your fix was instant. It got locked after the last software update-I saw the symbol in the top right of my display, but had no idea what it was.