DB_ERROR_NOT_CONNECTED occurs when something is wrong with your database.
Your /includes/configure.php and /admin/includes/configure.php files contain settings for the database (DB_SERVER
DB_SERVER_USERNAME, DB_SERVER_PASSWORD, DB_DATABASE, DB_PREFIX), which are used to log in to the database.
The 0 DB_ERROR_NOT_CONNECTED error occurs when those database settings no longer match the actual details for the MySQL server, the MySQL database, or the MySQL user+password, or if the database server is broken for some reason.
Work with your hosting company to fix the server.
Or, if you recently changed your MySQL user/database/password and didn't put those same changes into your configure.php files, you'll need to update your configure.php files to match what's required by your MySQL database server. Again, your hosting company can help with this.