Fixing broken Cortana on Windows 10 after upgrade to Version 1709.

Solution derived from
https://superuser.com/questions/947392/windows-10-search-cant-find-any-applications-even-calculator

Answer by Fleet Command.

This answer is my own discovery on Windows 10 Pro 1703 x64. (That's Creators Update, folks.) I had to resort to it because, to my surprise, the answer by Augusto Barreto did not work for me. Without further ado:

1. Do either of the following:
   • Create a new temporary user account with Administrative privileges, log out of your current account (very important) and log into the temporary account
   • Start your computer in Windows Recovery Environment
2. Delete, rename or move the following folder:

   C:\Users\[Your username]\AppData\Local\Packages\Microsoft.Windows.Cortana_cw5n1h2txyewy
   

   Note: In Windows Recovery Environment, C: can sometimes become D:, E: or even F:, depending on your computer's configuration.
3. Log back into your original account.

Cortana will not work immediately. (It is busy recreating the folder you just deleted.) But when it does, everything will be alright. Don't forget to delete the temporary user account.

Important notice: By the time I reached the conclusion to do this, I had ascertained that Windows Search Indexer was working okay and it was Cortana's problem that didn't show the result well. My evidence was that Cortana could find literally everything else (including my music) and searches from File Explorer worked well.

The difference, I did not create any temporary user.
Just delete Microsoft.Windows.Cortana_cw5n1h2txyewy directory then restart.