The warning that fills your screen is not from your computer
A page that shouts about a virus and shows a phone number is an advertisement for a scam. Real security warnings never ask you to call.
How the trap works
You open a web page and it suddenly fills the screen: a siren, a countdown, a message that your computer is infected and a number to call. It is only a web page. Its job is to frighten you into phoning people who will ask for remote access to your computer and for payment.
How to tell a real warning
Security messages from your operating system or antivirus appear in the system’s own windows, not in the browser, and they never include a phone number. Microsoft, Apple and Google do not ask you to call them from an error message.
Real updates arrive through the settings of your system or app. A web page that offers an “urgent update” or a “cleaner” is offering something else.
What to do
Close the browser. If the page will not let you, quit the browser from the task manager (Ctrl+Shift+Esc on Windows, Option+Command+Esc on a Mac) or restart the computer. Do not call the number and do not install anything the page offers.
If you already let someone connect to your computer, disconnect it from the network, remove the remote-access program, change your passwords from another device, and tell your bank if you paid.
This note is our own plain-language summary. For the full guidance, read the original: https://consumer.ftc.gov/articles/how-spot-avoid-and-report-tech-support-scams
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Real warnings do not come with a phone number.