These kind of pop-ups are fake alerts, trying to scam you into buying needless services/products.
Never ever call those phone numbers or fill in passwords etc.
If you can't close your browser in a normal way: Open Task Manager (press Ctrl + Shift + Esc on your keyboard) and in the tab Processes end all browser processes.
You can also try Shift + Esc to bring up Browser Task Manager and kill the popup without having to close the browser totally.
Or Windows key + X and then Task Manager etc
Also see: Open Task Manager in Windows 10 | Tutorials
And/or cutting the internet and/or shutting down your computer would stop all scam actions as well.
A good ad blocker should prevent pop ups/fake warnings/alerts/ads etc. Tracking blockers help too.
I use and recommend the free extensions uBlock Origin and Ghostery.
Most browsers offer the same or similar extensions...
Also go through your browser's settings and adjust them to your wishes. For example, block 3rd party cookies etc.
You might also want to reset your browser(s), see if you have any weird extensions/add-ons...
If needed, follow the steps provided here:
https://www.bleepingcomputer.com/virus-removal/remove-tech-support-scam-popups-in-your-browser
You might want to read:
How to tighten security and increase privacy on your browser - Malwarebytes Labs | Malwarebytes
Labs
and
Answers to common security questions - Best Practices
especially Beware of Phony Emails, Phone Calls & Tech Support Scams
which
- explains various methods how to get out of those fake alerts etc
- includes Resources to protect your browser, privacy & help prevent browser pop-up ads and scams
- explains
what to do when you keep on seeing these fake sites/getting pop-ups etc
despite having followed the above mentioned advices.
Also: