Can’t connect to my website…. HELP!

I’m getting this message when I try to log on to my website.

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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 mysql. 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?

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What the heck does this mean? I can’t really afford to have my website go completely down like this, and it isn’t the first time it’s happened. Can anyone tell me what I need to do to prevent this, or even who I need to contact for that matter? I run my website through yahoo hosting if that’s of any help.

Thanks!