well my phone does this constantly now. just go to phone and make it portrait mode (turn your phone upright, ull need to enable launch by turning under turning control in the phone menu) then press the app button and go to desktop, or simply close the phone app while still in portrait mode. Now its stuck in portrait mode.

to quickly go back to landscape, just open the phone app again while holding the phone in landscape position, lock the screen, slide to unlock and when the phone app goes back to landscape just press esc and everthing will be normal.