'Error establishing a database — Where your site stores its content and user data; overloaded or corrupted, it throws errors instead of pages. connection' is WordPress giving up because it can't talk to the database that stores every page, user, and order. You'll see a plain white page with that one line. It can be caused by a bad credential after a move, a database server that's down or out of space, or a database that's been corrupted — and the fix differs for each, so identify before you act.
Check these first, yourself
Check your host's panel: is the database server marked online, and how much of its disk quota — A cap on the disk, memory or emails your plan allows; hitting it is a common cause of sudden failures. is used? A full disk is a very common cause.
If you recently moved hosts or changed the site URL, suspect the credentials or hostname in the config file (wp-config.php) before anything else.
Don't start deleting plugins or rewriting files while you're guessing — you can turn a recoverable fault into real data loss.
If the database itself is damaged, stop writing to it and take a copy first; every further write reduces what can be recovered.
Should you fix it yourself or call a rescue?
Still have questions?
Did I lose all my content when this error appeared?
Almost never. The data is usually still on disk; the site simply can't open it. In most cases the database is repaired from a snapshot and content comes back intact — which is why stopping writes early matters.
Can my host fix it for me?
Hosts will confirm the database service is running and may restore a recent snapshot, but they won't repair a corrupted schema or untangle a bad migration. That's the part that needs the rescue.