Who I have the same questioned this solution

I updated my HTC One M8 to Marshmallow (Android 6) today.  The results have been fairly messy — quite a few problems with apps, and frustration with various aspects of the programming.  But the oddest moment so far has been that after doing a soft reset (turning the phone off and then on), I found the App Manager in the Settings to be completely empty.  Before I turned off the phone, the App Manager was full of apps.  Even now, both the home screens and the app drawer are full of apps, and the apps work.  A blank App Manager is something I've never experienced before, and I'm dreading the possibility that a factory reset will be needed.  Does anyone have a clue about what might be wrong, and how to fix this?