iPhone5 displays in portrait mode only regardless of lock setting

iPhone5 displays in portrait mode only regardless of lock setting. I'm not aware of any changes to settings that I have done that would cause this behavior to all of a sudden occur, but I cannot regain the ability to view in landscape mode. This occurs in all apps (e.g., photo viewer, Facebook, etc.). I'm probably missing something simple, but I'd sure appreciate some ideas on what's occurring.