If you have ever run into this ad you will know it immediately and I am sorry you experienced it, but that is exactly what the ad intended to do, scare the #$%^& out of you. This likely is not Malware or a hack attempt of your PC but rather a phishing attempt from an ad rotating on another person's site, even without their knowledge, if they use a third party ad service.
The ad has blaring alarms, flashing images and a loud voice WARNING you of danger to your computer.
I surf a lot on hundreds of TEs and run into this all the time while surfing on other sites even though I have excellent virus protection (more indication it is not a virus infection since it did not set off any of my virus alarms).
The solution I found that works best is to Control/Alt/Delete simultaneously and open Task Manager then close the browser. I never have to shut down my computer to stop it, just the browser and that MUST be done through Task Manager if you are using a Windows OS. I can't help you with Apple products.
With the browser closed AND my computer still running, I do a virus scan as a precaution. Then, I open the browser again for a new session and I do NOT restart the prior session.
A simple "fix" that takes 2 minutes because there is really nothing to fix but closing the browser through Task Manager wait a few seconds then open a new browser session.
Hope this helps some people.