Your site is down. First, check the database name, user, password, and server...

https://blogfreely.net/samuel-wang09/wordpress-database-connection-error-after-changing-the-table-prefix

Your site is down. First, check the database name, user, password, and server in WordPress settings. If they look right, inspect the database, repair tables, and review recent changes. Contact your host if the server is unreachable or the error continues.