WordPress cannot reach or authenticate to its database. There are four realistic causes.
1. Wrong credentials in wp-config.php
Open public_html/wp-config.php in File Manager and compare DB_NAME, DB_USER, DB_PASSWORD and DB_HOST against cPanel → MySQL Databases. Remember both the database and user names are prefixed with your cPanel username. DB_HOST should be localhost.
If you are unsure of the password, use MySQL Databases → Current Users → Change Password to set a new one and paste it into wp-config.php.
2. The user is not attached to the database
In MySQL Databases, scroll to Add User To Database, select the pair, and grant ALL PRIVILEGES.
3. A corrupted table
Add this to wp-config.php:
define('WP_ALLOW_REPAIR', true);
Then visit https://yourdomain.com/wp-admin/maint/repair.php and run the repair. Remove the line afterwards — leaving it in place exposes the repair page publicly.
4. Server-side issue
If the credentials are definitely correct and phpMyAdmin will not open the database either, open a support ticket — this may be a MySQL service issue on the server and we will check it immediately.