Final Update: We have resolved this issue which was found to be a result of an incorrect web application firewall setting blocking access to SSL certificates on proxy cPanel subdomains. At this time all access is restored.
Previously: We have put in a workaround to ensure cPanel access through our billing area continues to function while we investigate this issue.
We are currently investigating an issue where users are unable to access their cPanel. We'll update more as we get more information.