One thing worth trying is Control Panel -> Troubleshooter -> View All -> Search and Indexing. OnOn my Win10Windows 10 (just upgraded from Win8Windows 8.1), it found & fixed some permissions issues on search-related folders.
Of course it hasn't fixed my search problems of not finding most Start Menu apps &applications and settings (neither did reinstalling Cortana, messing with Index folder variations like just using Start Menu, rebuilding the index over and over, etc). SearchSearch worked mostly fine before the Win10Windows 10 upgrade - sigh.