Cannot imagine why it will not show the display (in photos, or email, or safari, etc.) in horizontal plane when I rotate phone. It always did before.
This may be due to having Orientation Lock enabled. This can be enabled or disabled from Control Center by swiping up from the bottom of your iPhone's screen.
iOS: Screen does not rotate
Do you see a small circular arrow around a tiny padlock in the upper right corner of your screen near the battery indicator? If you do then rotation lock is on. The previous post tells how to turn it off.
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