You may see an alert in the Safari web browser telling you to "Call Toll Free + 1-800-870-3001". This is referred to as browser hijacking – don't call!
A misspelled url or inadvertent click may send you to a website that immediately presents the window shown below:
This is a kind of browser hijacking, and the people that caused that message to display are not your friends. Calling that number will only lead to more problems and tremendous expense.
But don't worry, your machine is not really locked. Simply click the "Don't show more alerts from this webpage" box at the bottom left, then close the tab/window that remains. It too will display more warnings (as shown below), just ignore these too.
If that doesn't work, or you don't see the "Don't show more alerts from this webpage" checkbox, don't worry – just check out this post on escaping this browser hijacking attempt.