WordPress requires a stable link between PHP and the MySQL database to serve content. This error indicates your website’s code fails to authenticate or reach the database engine. It blocks all front end and back end operations. The failure happens before the theme loads.
Frequent triggers include incorrect login data in the configuration file or a crashed database service. High traffic occasionally exhausts server memory. This exhaustion forces the database to shut down. Corrupted database tables also prevent successful connections during the initial handshake.