Error establishing a database connection

Repair your database via PHPMYADMIN: https://www.a2hosting.com/kb/cpanel/cpanel-database-features/optimizing-and-repairing-mysql-databases-with-phpmyadmin

Make sure You have connected database Successfully .
Search your wp-config

// ** MySQL settings - You can get this info from your web host ** //
/** The name of the database for WordPress */
define( 'DB_NAME', 'database_name_here' );
/** MySQL database username */
define( 'DB_USER', 'username_here' );
/** MySQL database password */
define( 'DB_PASSWORD', 'password_here' );
/** MySQL hostname */
define( 'DB_HOST', 'localhost' );