Error with database

I cannot access my wordpress blog dashboard – one time I was able to access the dashboard but then could not access the post area or anything else.
I got an error message, as follows:

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?
If you’re unsure what these terms mean you should probably contact