WordPress site suddenly showing error establishing a database connection

also check disk space. a full hosting quota (usually old backup zips piling up) can stop MySQL and give you exactly this error. one minute to rule out.

did you change anything, or did it just happen? if it just happened, it's usually the database server rather than your site. check your host's status page or open a ticket, on shared hosting it's often MySQL down or your account hitting its connection limit (a bot hammering the site can do that). if you did change something (migration, password reset, new host), open wp-config.php and check DB_NAME, DB_USER, DB_PASSWORD and DB_HOST against what's in your hosting panel. one character off is enough. if the front end errors but /wp-admin says tables need repairing, add define('WP_ALLOW_REPAIR', true); to wp-config, run the repair page, then remove the line.