Master pages have disappeared from Project Setup but are still visible/being used by existing topics where they have been used.

Two of the three master pages contained in my project have disappeared from the Master Pages folder in Project Setup.  They are still "assigned" to the topics where they have been used, and those topics look fine, but I am unable to select them when creating new topics.  When I look at my local project files, I see them in the folder for this project so they are not completely gone.  I did try to re-import them into the Project Setup, but got no results.  How do I get them back into the Project Setup so they are available for use?  Thanks!