A good place to look is at the preferences. Move the preferences folder (found in the Output folder) to the desktop, then restart X‑Plane and default preferences will be restored. If the default preferences do not fix your problem, you can simply replace them with the folder you moved to the desktop and restore your personalized settings.
The final common culprit is third party add ons, such as scenery and plugins. Try moving the Custom Scenery folder and plugins folders (found in the Resources folder) to the desktop, then restart X-Plane.