2 days ago my iMAC was hit by a virus attak.The alert said to call a phone number within so many minutes or terrible things would happen.I did not call the number and sure enough, they did. My machine has been totally blocked from the internet. What do I do? I am using an iMac 21.5 in.(2013), OS10.14.6 and Safari 12.1.1. Firefox is also installed and it will not work either. Please help!