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Have you checked settings to make sure orientation lock hasn't been accidentally turned on? You may want to try powering your phone off and then back on to see if that helps too.
You can check to the lock by pulling up the quick menu at the bottom of the screen to avoid going into settings. The lock is the last icon at the top of that menu
Not to mention you should be able to see the icon at the top of your screen if it's active (sorry can't edit my posts on the mobile site it appears)
Not every app or screen can be viewed in landscape.
THe side switch can lock the phone in portrait view or mute phone, depending on what is chosen in settings.