1、 vim app/etc/local.xml
首先确定网址是:www.abc.com/admin
2、通过日志发现后台（Admin）的  store_id 和  website_id 应该设置为 0，所以我们需要工作修改相应的值，执行下面的SQL语句：
mysql -p 
SET FOREIGN_KEY_CHECKS=0;
UPDATE `core_store` SET store_id = 0 WHERE code='admin';
UPDATE `core_store_group` SET group_id = 0 WHERE name='Default';
UPDATE `core_website` SET website_id = 0 WHERE code='admin';
UPDATE `customer_group` SET customer_group_id = 0 WHERE customer_group_code='NOT LOGGED IN';
SET FOREIGN_KEY_CHECKS=1;
注：通过查看Magento系统日志：
2011-03-20T15:13:20+00:00 ERR (3): Recoverable Error: Argument 1 passed to
Mage_Core_Model_Store::setWebsite() must be an instance of Mage_Core_Model_Website,
null given, called in /var/www/html/yanggaojiao/shop/app/code/core/Mage/Core/Model/App.php
on line 591 and defined  in /var/www/html/yanggaojiao/shop/app/code/core/Mage/Core/Model/Store.php on line 292
2011-03-20T15:13:20+00:00 ERR (3): Recoverable Error: Argument 1 passed to Mage_Core_Model_Store_Group::setWebsite()
must be an instance of Mage_Core_Model_Website, null given, called in /var/www/html/yanggaojiao/shop/app/code/core/Mage/Core/Model/App.php
 on line 611 and defined  in /var/www/html/yanggaojiao/shop/app/code/core/Mage/Core/Model/Store/Group.php on line 227
完成后再次清空些站的缓存后台就可以进去了。