If your Zune HD screen is not rotating when you flip your screen from landscape to portrait or from portrait to landscape, the problem may be the accelerometer. If it is the accelerometer, you can fix this by resetting the accelerometer and then the screen should rotate normally.

To reset it, go to Settings from the home screen and select About. You can then select the Reset Accelerometer option.

You then need to lay your Zune HD down on a flat surface and tap the OK button. It'll go through the process of resetting. Don't touch your Zune or allow it to move during this time.

Once the reset process is done, tap OK and your screen should rotate again.