I have no idea why or what I have broken in the process. But here is what worked for me.
Kill process
Explorer.exe
via Task Manager.Delete this registry key.
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\FolderTypes\{ef87b4cb-f2ce-4785-8658-4ca6c63e38c6}\TopViews\{00000000-0000-0000-0000-000000000000}
- 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.