Problem fixed, thanks to Mal998!
Thanks to Mitch! We talked on the phone, and went through the process of deleting the affected sceneries from the library in the Settings menu of FSX. The problem was locating the files so I could use the Add Scenery function, and it took me a while to find them again.
But I did. What had happened was I had just added an external drive to my system, and had move a LOT of my files off of my internal drives, to save space. It was because of that - which caused the error. I had changed the address of the files.
All is okie dokie again.
Thanks Mitch, and thanks to everyone who suggested solutions!!
Pete