This is a bogus alert that seems to be hitting Mac users, and these people will not help you.
The damage gets done if you call them and they convince you to give them remote access to your computer so they can "help" you. In fact, all they do is help themselves to private information, and a couple hundred dollars if you then give them your credit card information.
If you find yourself locked up on that page:
Start Safari with the Shift key down.
Under the Safari Menu, Choose "Clear History and Website Data…"
Clear at least Today and Yesterday
Restart the computer.
Stick to a less unsavory part of the web.
If you have been subject to this phishing/harassment, please let me know, and let me know if this helped. I offer a free 50 minute multimedia presentation called Safer Computers, Tablets and Phones in Minutes to clubs and community/social groups in Massachusetts and Connecticut. Please email or call me if you think your organization would be interested