Error establishing a database connection

Hi, I’m new to all of this and trying to help update my friend’s site that is using an older version of wordpress and out of nowhere started getting this error:
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Error establishing a database connection

This either means that the username and password information in your wp-config.php file is incorrect or we can’t contact the database server at localhost:3306. This could mean your host’s database server is down.

Are you sure you have the correct username and password?
Are you sure that you have typed the correct hostname?
Are you sure that the database server is running?

If you’re unsure what these terms mean you should probably contact your host. If you still need help you can always visit the WordPress Support Forums.
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It is on the frontend and the backend. It happened the other day but popped back up after 20 minutes or so. But it happened again today and hasn’t come back yet. It is hosted through Linode but they said they don’t offer support and I am currently getting desperate. I am not sure how to resolve it if it is on my end and I don’t want to lose anything. Any help would be appreciated!