The most common reason for an unexpected screen rotation is that the ‘Auto-Rotate’ feature has been disabled. It is usually located in the ‘Display’ settings, under ‘Orientation’ or ‘Display’ tab.

It may be helpful to look for any available updates for the phone’s software. Check in the ‘Settings’ app, under ‘System Update’.

Perform a clean restart of the phone by pressing and holding down the ‘Power’ button for at least 5 seconds.

If the issues persists, it may be helpful to manually re-calibrate the gyroscope. On some phones, this be accessed in the ‘Settings’ app, under ‘Sensors’.