Virus/Malware went undetected by Norton

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During August, while accessing an appliance manual website (2x) and Amazon.com (1x), without Norton notice of an unsafe website: my computer locked up; screen flashed with rapidly repeating messages of infection. Without my ability to interrupt, I needed to turnoff my computer at the power switch. Norton scan was run after each occurrence and no threats were found by Norton.  What's going on? What's my next step to assess and protect my computer?

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Niko65:

[...] while accessing an appliance manual website (2x) and Amazon.com (1x), without Norton notice of an unsafe website: my computer locked up; screen flashed with rapidly repeating messages of infection. Without my ability to interrupt, I needed to turnoff my computer at the power switch. 

Alt + F4: The Windows keyboard shortcut for closing applications, explained 

  • Alt + F4 is a Windows keyboard shortcut that completely closes the application you're using.
  • It differs slightly from Ctrl + F4, which closes the current window of the application you're viewing.
  • Laptop users may need to press the Fn key in addition to Alt + F4 to use this shortcut.

If Alt + F4 doesn't work, you can close an active application using Task Manager. To do this, press Ctrl + Alt + Delete, then click Task Manager. Alternatively, press Ctrl + Shift + Esc to open Task Manager or right-click on the program you're trying to close and click "End task."

  

https://www.businessinsider.com/guides/tech/alt-f4#:~:text=
Note: businessinsider.com is ads supported

for example:

as always, your mileage may vary

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Do you run Norton Safe Web extension?
Do you run ad/content blocker? 

How to remove a fake virus alert: A step-by-step guide
https://us.norton.com/blog/emerging-threats/fake-virus-alert

Remove “Your Windows 10 Is Infected With Viruses” Pop-ups
https://malwaretips.com/blogs/remove-your-windows-10-is-infected-with-viruses-fake-alerts/

Remove “Warning! Your Computer Is Infected” fake alert (Support Scam)
https://malwaretips.com/blogs/warning-your-computer-may-be-infected-removal/

Please review: 

Can you post "pop up message' screenshot?
Please review: How to post an image in the forums
https://community.norton.com/en/forums/how-post-image-forums-0