Error Establishing a Database Connection

I have 3 websites that are hosted by BlueHost. My websites are http://www.agelesswayoflife.com, http://www.makingyourlivingonline.com and http://www.contentpals.com. Recently, I went to each of my website and I got this error for all of them: “Error establishing a database connection” I have not done anything on my websites. I’ve tried logging in to my WP Dashboard and I got this error:

“Can’t select database
We were able to connect to the database server (which means your username and password is okay) but not able to select the DB_NAME database.

Are you sure it exists?
Does the user DB_NAME have permission to use the DB_NAME database?
On some systems the name of your database is prefixed with your username, so it would be like username_DB_NAME. 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.”

My database name seems fine. I logged in to my cPanel and I checked my databases. They were all okay. As a matter of fact, my websites were up and running yesterday. I don’t know why this is happening. I don’t even know what the problem is. Please help me. Thanks.