Error establishing a database connection

The error message you're seeing indicates that your WordPress site is unable to connect to its database. This is a common issue that can occur for several reasons, even if your instances appear to be running normally. Here are some steps you can take to troubleshoot and potentially resolve the issue:

1. Check the MySQL/MariaDB service: The database service may have stopped running. You can try restarting it by connecting to your instance via SSH and running the command:

   sudo /opt/bitnami/ctlscript.sh restart mysql

   You can also check the status of the service with:

   sudo /opt/bitnami/ctlscript.sh status

2. Verify database credentials: Ensure that the database connection details in your wp-config.php file are correct. Sometimes, these can become corrupted or changed unexpectedly.

3. Resource constraints: Your instance may be running low on resources, particularly memory, which can cause database connection issues. Monitor your instance's resource usage and consider upgrading if you're consistently running low.

4. Check error logs: Review your Apache and MySQL error logs for any specific error messages that might provide more insight into the issue.

5. Database server accessibility: Ensure that your database server is accessible from your web server. If they are on separate instances, check the security group settings to make sure the proper ports are open.

6. Plugin conflicts: If you have recently updated any plugins, they might be causing conflicts. Try disabling all plugins temporarily to see if the issue resolves.

If these steps don't resolve the issue, you may need to investigate further by examining log files or considering a restoration from a recent backup if available. If you're not comfortable with these technical steps, it may be best to consult with a WordPress expert or your hosting provider for more detailed assistance.

Remember, it's crucial to regularly backup your website to quickly recover from such issues in the future.
Sources
Error establishing a database connection - Lightsail and Wordpress | AWS re:Post
Lightsail/Wordpress site mysteriously down and inaccessible | AWS re:Post