Did you tried deleting the WordPress database from mySQL and recreating a fresh new one and then install WordPress?
That would serve no purpose whatsoever. Anyway...
In most cases this is due to either the MySQL user not having the correct privileges, or the version of WAMP requires that you specify port 3306.
If you use the root user/password in wp-config.php, does WordPress load? If not, then change DB_HOST in wp-config.php from localhost to localhost:3306 and see if it works. If that doesn't work, then check the MySQL logs -- there should be something listed to help you locate the problem.
If the root user worked, then go to the Users page in phpMyAdmin. Make sure the user has full access (not just USAGE) to the specified database, and also make sure the user has host access to localhost and 127.0.0.1. That should be enough to get you working.