This is caused by deleting a scenery file from your hard drive without removing the reference to it in your scenery library first. In other words, your scenery library file thinks it still exists when in fact you deleted it. The easiest way to fix this is to put the scenery back on your hard drive, delete it from your scenery library, and then delete it from your drive again. You should be able to recover it from your recycle bin.
Come to think of it, although I don't think I ever tried this, you might just be able to load your sim and delete it from your scenery library list without having to reinstall the scenery.
Steve