Is rotation lock turned on on your phone? If so, swipe up from the bottom of your phone to bring up the control center and look for a round button with a lock on it and disable rotation lock.
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=== Update (01/13/2018) ===
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Is rotation lock turned on on your phone? If so, swipe up from the bottom of your phone to bring up the control center and look for a round button with a lock on it and disable rotation lock.
Is rotation lock turned on on your phone? If so, swipe up from the bottom of your phone to bring up the control center and look for a round button with a lock on it and disable rotation lock.