Auto-rotation

Make sure auto rotation is turned on from the notification bar. If it is already enabled, then you may want to reinstall the latest version of your stock firmware. Alternatively, you can try out different third apps that lets you auto rotate.

Did you try to reset your HTC?

Settings > Backup and Reset > Reset Phone > Ok > Ok.

Hey @Koch fe, before moving forward in factory resetting your device like most of the member have already guided you over here, Make sure the auto screen rotation is checked, head over to the settings and look for the option display and gestures tap over it and the very first option you will see is Auto-rotate screen, make sure that its's ticked and again check if the screen auto rotates !!

@Koch fe, if that's the case then you can move forward in performing factory resetting you can do that either by heading over to the

Tap Settings>Storage>Factory data reset>Reset phone>Erase everything.

or you can also factory reset your device by booting onto the boot loader menu

• Switch the device off,wait for a few seconds.
• Press and hold the VOLUME DOWN + POWER BUTTON button.
• Wait for the screen with the Android images to show up, and once it appears release the POWER and VOLUME DOWN buttons.
• Press VOLUME DOWN button to navigate to FACTORY RESET, and use the POWER button to select.
• Once the Factory resetting completes your device will reboot normally.

Note: Before moving forward with the factory reset make sure you have backup all of your data as it's going to wipe out all the data stored over your device.