The same thing happened to me last night but as the installation of '18 wiped '17 I could not try an earlier version.
Tonight I opened the same book file to work on a graphic placement and after doing what I had to thought I'd change to my 'indexing' workspace and try to work out what might have happened last night or what I might need to now do now that an upgrade's happened since I last worked on the index.
Lo and behold the index panel was how it was before, populated with all the references. I've got no idea of why it disappeared and has now reappered as my computer is only on when I'm sitting at the desk. Tonight's job was to try generating the index from scratch but thankfully I have no need to. Fingers crossed it doesn't happen again as this is the third book in a series of four.