Hello,

The “Error establishing a database connection” occurs when WordPress is unable to make a connection to the database. A number of things can affect your WordPress database connection.

This issue can be caused by incorrect database information in your WordPress settings, corrupt database, or an irresponsive database server. You can refer to our community article for more information

https://www.digitalocean.com/community/tutorials/how-to-debug-the-wordpress-error-establishing-database-connection

One of the most common reason is MySQL stopped running. It stopped or failed to start as a result of not enough memory.

A useful program to review the logs one page at a time is less, for example:

less /var/log/mysql/error.log

If you see memory issues such as mmap can’t allocate then you can be sure that you are having memory issues. You can choose to optimize your MySQL configuration, or use caching mechanisms to reduce the application’s load on MySQL. If nothing works, you can try resizing your server to a one with higher CPU and RAM.

Hope this helps!

Cheers,
Lalitha