Are you having issues with your site? Is it displaying a Sucuri page with the backend timeout message like this?
There are two things that could be causing this:
1 - Your hosting server is offline
What to do? Check with your hosting provider if the server is up, having resource issue or if they have changed your hosting IP.
2 - The hosting server is blocking us via firewall or other security tool like mod_security
To solve this issue, please contact your hosting provider and ask them to whitelist our IPs. You can find the list of IPs used by us on your dashboard. To find those IP addresses:
Click the settings gear icon beside the affected domain and select General
Click the List Firewall IP Addresses (for whitelisting) link
If even after that your site is not responding open a support ticket so we can further investigate the issue.