Fixing 403, 404, 500 and 503 errors

403 Forbidden

• No index page: public_html must contain an index.html or index.php.
• Wrong permissions: folders should be 755 and files 644. See File and folder permissions explained.
• A rule in .htaccess or the firewall: rename .htaccess temporarily to test. If the firewall blocked you, see the web application firewall.

404 Not Found

• Check the file exists, with exactly the same name. Names are case-sensitive: About.html is not about.html.
• If only the home page works on a WordPress or similar site, set the web server to Nginx & Apache so .htaccess rules work. In WordPress, also go to Settings → Permalinks and click Save Changes.

500 Internal Server Error

• A PHP error is the most common cause. Show the error to find it.
• A mistake in .htaccess: rename it temporarily to test.
• A plugin that is not compatible with your PHP version: switch back to the previous version.

503 Service Unavailable

• Maintenance mode may be on. See Putting your website into maintenance mode.
• Your site may have hit its resource limits. Check the dashboard and see Understanding your plan limits.

"This site can't be reached"

This is a DNS problem rather than a website problem: the domain does not point anywhere yet, or the change has not spread. See Pointing your domain to Twerple.