“Cortana and Start menu are not working”.
Googled on another machine and found that uninstalling AVAST was a solution.
Indeed it was . Given the spam popups and this, Avast is gone gone gone. I’ve been a user for many many years.
Doing a littl research would have revealed that this is a well known Windows 10 problem.
Simply booting into Safe Mode with networking and then rebooting into regular mode would have fixed the
Windows 10 Cortana and Start menu problem.
Nothing is perfect, especially not Windows.
I don’t know how you have searched, but it seem you did not do it correctly.
As Bob said, it is a (very well) know issue with Windows.
It happens with all kinds of software, not just avast.
Microsoft has admitted the problem and is working on it.
Knowing Microsoft they probably will release a good fix close before the EOL of Windows 10 ;D
That did not work.
The research I did do showed that you’re right, it is a well known problem and removing Avast fixes it.
No, removing avast is not fixing it.
Just because you don’t see it, it doesn’t mean the problem is fixed.
The bug/problems are still there and MS need to fix them.
If you ran out of fuel, you can use another car.
Although you can drive again, the problem is not solved.
The initial car still has no fuel and will not drive. 
Time will tell.
So far Panda is working well on this machine, while Avast still lives on my other non - 10 machines.