I have no idea why or what I have broken in the process.
But here is what worked for me.

1. Ctrl+Shift+Right-click on an empty part of the taskbar and clicking "Exit Explorer" (or kill it via Task Manager if that doesn't work).

2. Delete this registry key.

>HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\FolderTypes\\{ef87b4cb-f2ce-4785-8658-4ca6c63e38c6}\TopViews\\{00000000-0000-0000-0000-000000000000}

 1. Start process `Explorer.exe` via Task Manager.

**This is one of many answers posted here. Feel free to try other people's suggestions.** If you combine all answers in one I'll accept your answer.

[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/bqZyX.jpg