Critical warning Safari message
I just received a "Critical Warning that my Mac may be compromised" and that I need to contact 1(855)244-7750 for immediate assistance. I immediately shut down computer, but now when I re-enter Safari, I'm locked out and the warning message just keeps popping up. Is this valid and if so, how do I fix?
iMac, OS X Mavericks (10.9.5)