How to Reset a Forgotten Root Password

If you never set a root password for MySQL, then the server will not require a password at all for connecting as root. It is recommended to always set a password for each user. See section 4.2.2 How to Make MySQL Secure Against Crackers.

If you have set a root password, but forgot what it was, you can set a new password with the following procedure:

Take down the mysqld server by sending a kill (not kill -9) to the mysqld server. The pid is stored in a `.pid' file, which is normally in the MySQL database directory:
shell> kill `cat /mysql-data-directory/hostname.pid`

You must be either the Unix root user or the same user mysqld runs as to do this.
Restart mysqld with the --skip-grant-tables option.
Set a new password with the mysqladmin password command:
shell> mysqladmin -u root password 'mynewpassword'

Now you can either stop mysqld and restart it normally, or just load the privilege tables with:
shell> mysqladmin -h hostname flush-privileges

After this, you should be able to connect using the new password.
Alternatively, you can set the new password using the mysql client:

Take down and restart mysqld with the --skip-grant-tables option as described above.
Connect to the mysqld server with:
shell> mysql -u root mysql

Issue the following commands in the mysql client:
mysql> UPDATE user SET Password=PASSWORD('mynewpassword')
    ->             WHERE User='root';
mysql> FLUSH PRIVILEGES;

After this, you should be able to connect using the new password.
You can now stop mysqld and restart it normally.