There has been reports that a nasty malware infecting Safari has been released in the internet.
As the picture shows an alert and a phone number is displayed for you to call a support number.
Most of the time the 800 number is a bogus company that would sell you security software that you do not necessarily need because the error was generated by the scamware developers. Basically it is a non existent error.
Most of the time it is a pop-up error message that reads in part “Your Apple Computer has been blocked. Mac iOS alert! System might be infected due to unexpected error! Your Browser might be hijacked or hacked.”
It is a very common form of malware to scare people every time they open the browser.
There is no way of resetting Safari outside of Safari. Even though deleting some of Safari’s preference files might help, most of the time it does not.
Use a different computer to download cleanup tools such as Malwarebytes Anti-Malware for Mac.
After you have ran the cleanup tool like Malwarebytes and it has done its job, it would be wise to install a secondary browser like Firefox or Chrome.
Upgrade to the latest version of OS X El Capitan or Sierra if possible.
Never trust a number that appears in a pop-up screen from your computer. It would be safer to have it checked by an Apple professional from the Apple Store or a local technician in your community. Calling Apple support would be one of the other options.