Can’t select database

I have a high traffic wordpress site. We get around 250,000 unique visitors monthly. We have an issue with our site and it says:

We were able to connect to the database server (which means your username and password is okay) but not able to select the weusecou_wp database.

Are you sure it exists?
Does the user xxxxxx_wp have permission to use the xxxxxx_wp database?
On some systems the name of your database is prefixed with your username, so it would be like username_xxxxxx_wp. Could that be the problem?

If you don’t know how to set up a database you should contact your host. If all else fails you may find help at the WordPress Support Forums.

This site was working just fine this morning, and all of a sudden it’s not working. This is not a fresh install, this is an established site. We do have a separate database and web server if that matters. What are some ways to trouble shout this?