I'm not sure why the pages would have performed a disappearing act on you. Your project isn't being stored and edited while it's on a network drive is it? If so, that could explain it.
To get the pages listed again I might suggest you try the following approach.
Create a new master page using a name you have never used before. Perhaps name it Boffo.
Right-click the Master Page and choose Explore.
Close the project.
From Windows Explorer (which should have opened to the project location earlier) locate a file named rhtemplate.apj.
Open this file for editing using Windows Notepad.
It should look something like this:
Select from <file> to </file> and copy it. Then paste for as many files as you have.
Change the file names from Boffo.htt to what the real file names are.
Close and save the file.
Re-open the project and see if your Master Pages are back. If not, you may need to repeat these steps and delete the .CPD file before restarting RoboHelp.
Adding the master page file names back into the rhtemplate.apj file did it. The master pages are showing up again. Don't know what happened there. Weirdness!