让mysql不载入权限表，命令：mysqld --skip-grant-tables

命令：mysqld --skip-grant-tables（windows）
mysqld_safe --skip-grant-tables user=mysql或者/etc/init.d/mysql start --mysqld --skip-grant-tables （linux）

Windows平台例子:
C:\Users\duansf>mysqld --skip-grant-tables

打开另外一个命令窗口，进入mysql的交互界面
C:\Users\duansf>mysql -uroot -p
Enter password: ******
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 15
Server version: 5.5.13 MySQL Community Server (GPL)

Copyright (c) 2000, 2010, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.


Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.


mysql>

用update语句重置root密码。
mysql> show databases;
+--------------------+
| Database|
+--------------------+
| information_schema |
| mysql |
| performance_schema |
| test|
+--------------------+
4 rows in set (0.07 sec)

mysql> use mysql
Database changed


mysql> update user set Password=password('123456') where user='root';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0


mysql> commit;
Query OK, 0 rows affected (0.00 sec)


mysql> flush privileges;
Query OK, 0 rows affected (0.24 sec)


重新登陆验证密码修改是否成功
C:\Users\duansf>mysql -u root -p123456
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 16
Server version: 5.5.13 MySQL Community Server (GPL)

Copyright (c) 2000, 2010, Oracle and/or its affiliates. All rights reserved.


Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.


Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql>