Well done for being suspicious, you would be amazed how many users fall for this!
This is a well known Tech Support Scam do not click anything on that page and do not phone any number on that page.
When this happens, open Task Manager (Ctrl + Shift + Esc)
On the processes tab, locate your browser and end task on that
You can then continue to user your PC and browse the Internet
This is caused by a browser re-direct on a page you are visiting, try to avoid any page that jumps out to this sort of page.