If you haven't editied the username, password or hostname in your /conf/_config.php file, then number three is probably the issue. You'll have to contact your web host and see if there's a problem with the mysql server.

I recently ran into the same problem. Frustrated the heck outta me, too. I spent a few hours trying to figure out what the heck was going on. I figured out that it had something to the user on the database. I created the user for the database, but failed to ADD the user the specific database. After I added the user to the database I was working with, it worked great. :) Hope this helps someone out.

Jamie