If you are just visiting the site, just wait a bit and it should be back soon. We apologize for the inconvenience.
If you own the web site, please verify with your hosting company if your server is up and running and if they have our firewall IPs whitelisted. Please wait a few minutes and try again. If the problem persists, open a ticket on our support page https://support.sucuri.net and we will assist with troubleshooting.