A virus warning has taken over the screen
If a warning fills the screen and gives you a phone number to call, it is a scam. Your computer is almost certainly fine.
What's usually going on
These warnings are web pages or browser notifications dressed up to look like Microsoft, Apple, or your antivirus. They are loud, they often lock the browser, and they always push you to call a number or install something. Real security software never asks you to phone anyone.
Worth trying first
Don't call, don't click, don't pay.
Do not call the number, do not click anything inside the warning, and never let a stranger connect to your computer.
Close the browser the hard way.
On a Mac, press Command, Option, and Escape together, choose the browser, and click Force Quit. On Windows, press Control, Shift, and Escape, choose the browser, and click End task. If that fails, hold the power button until the computer turns off.
Reopen without restoring.
When you open the browser again, decline any offer to restore the previous pages, or the warning comes straight back.
Switch off the notifications behind it.
If small warnings keep appearing in the corner of the screen, a website has been given permission to send notifications. In the browser's settings, search for "notifications" and remove any site you don't recognize.
When to hand it over
If you called the number, let someone connect to your computer, or gave any card or bank details, treat it as urgent. Disconnect the computer from the internet, call your bank using the number on your card, and get in touch. We'll remove anything that was installed, change the passwords that matter, and check nothing has been left behind.
Your own technologist, on call.
Tell us a little about your setup. We'll take it from there.