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Check the contents and permissions on your cache folder. If you find a file called adminmenu.php and the permissions are set to rwx-rwx-rwx on the cache directory, then delete adminmenu.php, logout then log back in. The file should be recreated and your menu restored. If the file is not present and your folder permissions are not rwx-rwx-rwx, try changing your folder permissions then log out and back in.
On some servers, you can use permissions as strict as rwx-rx-rx on the cache and templates_c folder, but this is not common.
Hope that helps.
James
Insanity can be defined as "doing the same thing over and over and expecting different results."
Stupidity is not a crime. Therefore, you are free to go.