Database Error When Attempting to Open Site

I just tried to open my site and received this message:

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. 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?

Yesterday, I backed up my database, and I’m afraid that somewhere along the way, I caused this error. I followed the WP steps to backing up, and didn’t mess with anything else. I don’t understand why I can’t open my site now. Please help…Thanks!