Acrobat Reader DC only showing a few pages, not all pages??

Hi Community,

I opened a .pdf file in Reader DC, and for a brief moment, it showed me 17 pages in my .pdf document, via some sort of a table of contents or icons on the left hand side of my document in a pane.  I could click on these icons and it would show me every single page, many pages I didn't even ever know where in here because I never saw them before.  It is NOT the thumbnails, it was some other tool.  (Table of contents or something?)

I accidentally closed the document, and when I re-opened it again, I only see thumbnails with only 6 pages.  Where did the other 11 pages go?  Why are they hidden?  How did they hide themselves?  Why did they hide themselves?  The thumbnails only show me the first 6 pages.  What do I click on to unhide or show the other 11 pages?  I have been struggling with this for almost 2 hours, and nothing on Google, and nothing on Adobe help at all.  Where did these pages disappear to????

The document is my full tax return, which had 17 pages in it.  Now Acrobat Reader DC shows the first 6.  I can't see any of the other pages anymore.  HELP!!

Thanks