This message means your website files loaded fine but could not talk to the database. There are only a handful of causes, and they are easy to work through in order.
1. Wrong credentials. By far the most common, and it usually appears right after a migration or a password change. Open your config file and compare every value against cPanel, character by character. A trailing space counts.
2. Host is not localhost. If a migration tool wrote in a hostname or IP from the old server, change it back to localhost.
3. The user is not attached to the database. Both can exist without being linked. In MySQL Databases, scroll to Add User To Database and confirm the pairing, with ALL PRIVILEGES.
4. A corrupted table. Rarer. In phpMyAdmin select all tables, then choose Repair table from the dropdown at the bottom.
5. The account is over its resource limits. If your site is being hammered, MySQL connections get refused. Check Resource Usage in cPanel - if you are hitting the ceiling regularly, the fix is a bigger plan rather than more debugging.
Still stuck after those five? Open a ticket with the exact error and what changed most recently. That second detail saves everyone a lot of time.
