Error establishing a database connection

Todo dia quando verifico meu site aparece essa mensagem, porém, não mudamos nada ou fizemos alguma coisa para isso acontecer. Simplesmente aparece, e para resolvermos temos que simplesmente reiniciar o servidor que o site volta ao normal. Porém, é um pouco tanto incomodo ter que ficar reiniciando o servidor todas as manhãs, além de ter a chance de passar o final de semana com o site fora do ar (como aconteceu nesse final de semana). Gostaria de ajuda de vocês. Please.

Mensagem do erro: “Warning: mysqli_real_connect(): (HY000/2002): Can’t connect to local MySQL server through socket ‘/var/run/mysqld/mysqld.sock’ (2) in /var/www/html/wp-includes/wp-db.php on line 1490

Deprecated: mysql_connect(): The mysql extension is deprecated and will be removed in the future: use mysqli or PDO instead in /var/www/html/wp-includes/wp-db.php on line 1520

Warning: mysql_connect(): Can’t connect to local MySQL server through socket ‘/var/run/mysqld/mysqld.sock’ (2) in /var/www/html/wp-includes/wp-db.php on line 1520
Error establishing a database connection
This either means that the username and password information in your wp-config.php file is incorrect or we can’t contact the database server at localhost. This could mean your host’s database server is down.

Are you sure you have the correct username and password?
Are you sure that you have typed the correct hostname?
Are you sure that the database server is running?”