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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.

Method for Creator's update Check out this answer.