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Press Command, Option and Esc keys and this will bring Force Quit window. Select Safari and click Force Quit. This will close Safari.
Then hold down Shift key on your keyboard and open Safari. This will open Safari without loading that website. Then click on Safari menu and select Clear History and Website Data. On the dropdown next to Clear, select All History and click Clear History. ( If you see Reset Safari there, then tap Reset Safari and reset it - only on old version of OS X)
Then click on Safari menu and select Preferences. Then click on Privacy tab. Click Remove All Website Data. Click Remove Now. Restart Safari.
Then download, run and scan your Mac with this application, Malwarebytes --> https://www.malwarebytes.org/mac-download/
This will find and remove all adwares/malwares from your Mac if any.
Please let me know if that didn't work, so we can continue troubleshooting.
You can also do this. Disconnect your Mac from WiFi or disconnect ethernet cable. Open Safari and this will prevent the page from loading. Then clear Safari. Connect it back to internet.