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Either update your rotation on/of setting in the setting control feature or reboot as a last resort
support.apple.com/kb/HT4085 said:Troubleshooting Rotation Lock
If your iPad display does not rotate as expected, verify that Rotation Lock is off:
1. Check the status bar for the Rotation Lock icon.
2. Toggle the Side Switch to verify that the device is not locked.
Note: Some apps do not rotate to match the current or locked orientation. Use the Home screen to determine Rotation Lock status.
3. If Rotation Lock is off and you are still unable to rotate the iPad display, restart your iPad:
Hold the Sleep/Wake button until "slide to power off" appears, and then slide to turn off your device. When it is off, press the Sleep/Wake button to turn it back on.