Deleting the Preferences
From X-Plane Wiki
Deleting X-Plane's preferences is an easy way to reset the simulator to its default settings after a setting has been changed that caused X-Plane to crash. It is also a good idea to delete the preferences after updating the simulator, due to the fact that the preferences files are continually changing.
To delete X-Plane's preferences:
1. Open the X-Plane 9 installation directory in My Computer, the Mac Finder, etc.
2. Go to the Output folder, then open the Preferences folder.
3. Delete the files within this folder (not the folder itself) to reset X-Plane’s preferences.