hi and thanks for your reply.
why root ? because there is no other user to login mysql, and root user has a password unknown, and phpmyadmin need the root password for his installation.
sudo /etc/init.d/mysql restart does not work, the correct command is service mysql start or stop.
i really don't know what happened but i just turn on the server today and mysql is running fine, i have changed the password with the same code.
http://www.sururi.com/2011/02/change-mysql-password-on-zentyal.htmlbut i use "service mysql" instead of "/etc/init.d/mysql".
i have created a database and i can access with the root user, i will create another user with less privileges, but at this time the thread is solved.
thank you =D