WordPress Database Error –

First thing you should do is to make sure that you are getting the same error on both the front-end of the site, and the back-end of the site (wp-admin). If the error message is the same on both pages “Error establishing a database connection”, then proceed onto the next step. If you are getting a different error on the wp-admin for instance something like “One or more database tables are unavailable. The database may need to be repaired”, then you need to repair your database.

You can do this by adding the following line in your wp-config.php file. Add it just before ‘That’s all, stop editing! Happy blogging’ line wp-config.php.

Once you have done that, you can see the settings by visiting this page: http://www.yoursite.com/wp-admin/maint/repair.php

Referenced: https://www.wpbeginner.com/wp-tutorials/how-to-fix-the-error-establishing-a-database-connection-in-wordpress/