How to access the administration panel if you cannot login again? I have created a custom theme. I have added the region user account menu in footer, but I forgot to add the region primary admin access.

Unfortunately I clicked on the logout button, and I am not able to log in again. The page at /user/login appears empty. Now how can I access the administration page?
You change your default theme in setting.php.Uncomment this line in your setting.php file 

 $conf['theme_default'] = 'garland';