Classic Shell Start Button Replaced By Win10 Start Button

I've been using Classic Shell for a few years on my Windows 10 system.
Today after being away from the computer I see that the Start button is no longer Classic Shell but is the Windows 10 Start button. How can I get my Classic Shell Start button back?

 

You have probably been upgraded to Win 10 version 2004 from version 1909. That would replace anything you have overridden with Win 10 defaults. If it's still available - (it isn't in development any longer) - you will need to reinstall Classic Shell.

 

So I clicked on c:\program files\classic shell\classicstartmenu.exe and that restored the Classic Shell Start button. When I shut down the computer and then restarted it, the Classic Shell Start button was still there and everything is back to the way I wanted it.
Still don't know why the Windows start button started appearing. As far as I can recall, the last Windows update occurred 3 or 4 weeks ago.

 

OK, you are still on your previous version. I should have asked you to hit WinKey + R and type winver before assuming you have been upgraded to the latest version, currently being pushed out.

 

Just wondering--what's the difference between the Classic Shell and the Win10 Shell? I might downgrade.

 

Dunno. I used to use Classic Shell, then I loaded Open Shell and had a few problems with it so I reverted to Classic Shell. Never heard of Win10 Shell.