Start with the Logs tab. It answers most questions faster than guessing.
The site shows a blank white page
A PHP fatal error. Open Logs and choose the PHP tab. The last entry names the file and line.
Do not switch display_errors on to diagnose this on a live site — the log has the same information without showing it to visitors.
502 or 504
PHP-FPM is not answering. Usually the script exceeded max_execution_time, or memory ran out. Check the PHP log, then consider raising memory_limit or max_execution_time on the PHP tab.
403 Forbidden
Usually permissions. Files should be 644 and folders 755. The Files tab shows and changes permissions.
It can also mean there is no index.php or index.html in public_html.
404 on every page except the home page
An application using pretty URLs that expects Apache rewrite rules. Container Hosting runs nginx, so .htaccess files are ignored. Your application needs its nginx-compatible configuration, or its own routing.
The domain does not load at all
Check the Domains tab first. If the DNS badge does not show the domain resolving to your container, the problem is at your DNS provider, not on the server.
A recently changed record can take a few minutes.
HTTPS has not appeared
Certificates are issued on the first https visit, and only once DNS points here. Confirm DNS first, then visit the site over https once.
The site is suspended
A suspended container serves a notice instead of your site. This follows an unpaid invoice — check Billing in your client area. Your files and databases are untouched and return as soon as the invoice is paid.
Still stuck
Open a support ticket and include the domain, what you expected, what happened, and the relevant lines from the Logs tab.