Many third party payware aircraft require activation. I know the 777 Worldliner is one of those. If you have reinstalled X-Plane or updated the plane, it may need to be activated again.
Unfortunately there is no one-size-fits-all solution if you have many planes from multiple developers. If you are having problems with your activation codes, please see documentation that came with the aircraft, or contact the developer of the plane to see if it can be reset.