I am using an iPhone 5 and am having the same problem, I cannot get from portrait mode into landscape mode anymore. In the status display (top screen line) the little @ symbol shows that I am locked in portrait mode, but I can't make it go away, and cannot view videos with the VLC app in landscape mode. When I double click on the home button and "scroll to the left" I just see all active apps displayed next to each other like playing cards), and if I terminate all other apps first and then double click on home, the standard display (4x6 app selections screen) just shrinks a bit and I cannot scroll left.
NOTE: I have just solved this problem by reading similar questions here, it is necessary that I swipe upwards, then I see the @ symbol in red and can tap it and turn it off, problem solved. I will leave problem and solution here in case some else can profit from it.