Broken Admin Menu, How Can I Repair It?
Unfortunately—and this is not an ideal solution, but seems to be what everyone is using successfully—I have had to use the following brute-force SQL statement in the past. 

First, backup your entire Drupal database!

Then, execute this in your favorite SQL manager:

DELETE FROM menu_links WHERE module = 'system';


Then visit /admin/config/development/performance and clear the cache.


taken from https://drupal.org/node/865702#comment-4015758