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Hi,
I had the same problem after I installed the protector module. If you installed that module and modified your mainfile.php, then you need to deactivate the string in the mainfile.php that you had to add as recomended by the protector developer. Edit mainfile.php.
If you can access admin via Firezilla, then just uninstall the protector module. You might still need to edit the mainfile.php
Also, make sure to check your files permission because I found that some of them had changed to 644 and it was blocking me from access (the same problem you had, OK webmaster but went back to log in page with no admin menu)
After I fixed the mainfile.php and the files' permission, I was able to get into the admin section via Firefox (Mozilla) just typing the URL to the admin directly. Then, I did some more housecleaning.
In IE and clone browsers, make sure that you empty the cache and temp files. Close the browser afterwards, and try again.
Hope this helps.