How do I fix "Error establishing a database connection" in WordPress?

WordPress could not connect to MySQL or MariaDB, or the database is damaged. Check in this order.

1. Is the database server running? On a VPS:

sudo systemctl status mysql      # or mariadb
sudo journalctl -u mysql -n 50

A common cause is the process killed by out of memory (check dmesg | grep -i oom). Add swap or lower innodb_buffer_pool_size and restart.

2. Are credentials right? Test the same values from wp-config.php manually:

mysql -h DB_HOST -u DB_USER -p DB_NAME

If this fails with access denied, the password changed or the user lost privileges. Reset: ALTER USER 'user'@'localhost' IDENTIFIED BY 'newpass'; and FLUSH PRIVILEGES;, then update wp-config.php.

3. Is DB_HOST right? localhost uses a Unix socket; 127.0.0.1 uses TCP. Some hosts need a specific hostname or port (host:3307).

4. Disk full? df -h. A full disk stops MySQL from writing and may crash it.

5. Corrupt tables. Add to wp-config.php temporarily:

define( 'WP_ALLOW_REPAIR', true );

Then open /wp-admin/maint/repair.php and run repair. Remove the constant afterwards, since that page needs no login.

6. Too many connections. Traffic spikes or bots can exhaust max_connections. Cache pages and block abusive IPs.

If only wp-admin fails but the front end works, the siteurl in the database or a plugin may be the problem rather than the connection. Always check the MySQL error log first; it usually names the exact reason.