You PC just froze. An official-looking message that appears to be from Microsoft pops up and tells you to call a support number. If this scene is familiar to you, you’ve been witness to a scam.
A compromised ad injects malicious code that hogs up all of your CPU, making it impossible (or painfully slow, if you’re lucky) to do anything on your computer. Closing the warning won’t fix the problem, but if you’re able to launch Task Manager you can kill the malicious code in the browser.
If you do opt to call the number, you’ll get an ever-so-helpful tech who will make the problem go away… for a few hundred bucks.