Your site is down. First check the database details in wp-config.php and check...

https://www.bookmarking-fox.win/seeing-error-establishing-a-database-connection-on-wordpress-first-check

Your site is down. First check the database details in wp-config.php and check for a hosting outage. If those are right, repair the database and review server logs. Contact hosting support if the database service is unavailable or you cannot access it.