1) Due to incorrect connection parameters
These parameters are located in the wp-config.php file. You can find it in the root folder of your WordPress installation. The credentials are database name, database user, and password. Check if they match the information, located in your cPanel or in case you remember database user and password, just reset them
If you forget them, it’s necessary to delete and recreate the database user. Use the password from the wp-config.php file. Now, refresh and check your site.
2) Due to corrupted database tables
Another reason for 343: "Error Establishing a Database Connection" could be corrupted database tables. You should check and repair them from the cPanel.
3) Due to many concurrent database connections
Too many connections can cause the 343 error, too. The default limit for most of the servers is 30 connections at once. If this is your reason for the error, then stop it by enabling caching plugins on the website. Due to this setting, the overall number of database interactions will be limited.
Finally, we should note, that 343: "Error Establishing a Database Connection” could be caused by other reasons such as log out of WordPress admin dashboard when it’s out of use, disabling the WordPress autosave or limits of WordPress revisions.