WordPress的权限是个好东西。但是今天一不小心把所有的权限都删除了。意思就是：
任何账号都不能登陆后台了。

解决方法：

可以通过在主题的functions.php里添加管理员权限。
提示一点：下面的代码先删除了管理员权限，再添加的管理员权限。
这是为了避免新添加的权限无法覆盖现有的管理员权限。如果使用下面的代码之前，自己试着添加过一些管理员权限，那么需要先清除。

if (get_role('administrator')) {
remove_role('administrator');
}
if (!get_role('administrator')) {
    add_role(
        'administrator',
        'Administrator',
        array(
            'create_sites' => true,
            'delete_sites' => true,
            'manage_network' => true,
            'manage_sites' => true,
            'manage_network_users' => true,
            'manage_network_plugins' => true,
            'manage_network_themes' => true,
            'manage_network_options' => true,
            'upload_plugins' => true,
            'upload_themes' => true,
            'upgrade_network' => true,
            'setup_network' => true,
            'activate_plugins' => true,
            'create_users' => true,
            'delete_plugins' => true,
            'delete_themes' => true,
            'delete_users' => true,
            'edit_files' => true,
            'edit_plugins' => true,
            'edit_theme_options' => true,
            'edit_themes' => true,
            'edit_users' => true,
            'export' => true,
            'import' => true,
            'install_plugins' => true,
            'install_themes' => true,
            'list_users' => true,
            'manage_options' => true,
            'promote_users' => true,
            'remove_users' => true,
            'switch_themes' => true,
            'update_core' => true,
            'update_plugins' => true,
            'update_themes' => true,
            'edit_dashboard' => true,
            'customize' => true,
            'delete_site' => true,
            'moderate_comments' => true,
            'manage_categories' => true,
            'manage_links' => true,
            'edit_others_posts' => true,
            'edit_pages' => true,
            'edit_others_pages' => true,
            'edit_published_pages' => true,
            'publish_pages' => true,
            'delete_pages' => true,
            'delete_others_pages' => true,
            'delete_published_pages' => true,
            'delete_others_posts' => true,
            'delete_private_posts' => true,
            'edit_private_posts' => true,
            'read_private_posts' => true,
            'delete_private_pages' => true,
            'edit_private_pages' => true,
            'read_private_pages' => true,
            'unfiltered_html' => true,
            'unfiltered_upload' => true,
            'edit_published_posts' => true,
            'upload_files' => true,
            'publish_posts' => true,
            'delete_published_posts' => true,
            'create_posts' => true,
            'edit_posts' => true,
            'delete_posts' => true,
            'read' => true,
        )
    );
}

这样，管理员权限是回来了。记住，权限回来了，就将上面的代码删除，不要留在functions.php里。否则会影响权限相关插件的使用，尤其User Role Editor。