I had the same issue after trying to update the PA-44 in the Marketplace. Did a search and found the post below by SuperSixBravo. After following his instructions, the problem was fixed and the plane appeared in my menu again. One thing I did differently is move the PA-44 file to a BU file instead of deleting it. BTW, thanks SuperSixBravo!
SuperSixBravo
"Go into your folders where you installed it. You need to find a folder that contains two folders one called Community and one called Official.
Go into the official folder and look for a folder that contains the name of plane. It will be obvious. Delete that folder and then restart msfs (I mean delete the folder that is the plane)
Once msfs has been restarted, go in to your content manager and download it afresh.
I would advise restarting msfs after it has installed and then all will be ok."