Is the missing content still present in the dashboard?
Were you using a modified version of the default or classic theme? These get overwritten on upgrade unless the theme’s directory name and theme name (in style.css) is changed.
Missing content is still present in the Dashboard.
I am using a modified default theme, but the modifications that I a aware of are one changed image using same file name, a little CSS alteration, Google analytics in the footer. At least that’s all the code changes I have done. The rest is all done in the Dashboard and its sub-pages.
So in the Dashboard, these elements show to be “Visible=Yes” and when I add a Category (for instance) it remains visible in the Dashboard but never on the pages or posts.