Rotate Screen with a Keyboard Shortcut
Hit CTRL + ALT + Up Arrow and your Windows desktop should return to landscape mode.
Rotate Screen with Display Settings Menu
If, for some reason, the shortcut keys don't work (and they should), try this method.
1. Right click on the desktop and select Display settings.
2. Choose a screen to fix if you have multiple monitors attached. Skip this step if you have only one.
3. Select Landscape from the orientation menu
4. Click Apply (or OK)
5. Click Keep Changes when prompted.