First confirm whether the problem affects one device or network, or all visitors. Check that the domain is active, nameservers and DNS records are correct, the hosting service is active, and SSL has not expired. Try another network and clear local DNS or browser caches.
In cPanel, check disk usage, error logs, file permissions, the document root, and recent application changes. For WordPress, a faulty plugin or theme may cause an error. If the issue continues, open a support ticket with the domain, exact error, time observed, affected networks, and recent changes.
