Cant login to admin

I recently had difficulty in logging in, but after much perseverance managed it. I then tried to auto update to the latest version 3.5.1 the installation failed since when I cannot log back in and receive the following error message Fatal error: Call to undefined function wp_is_mobile() in /users6/domains/findyourself.gr/web/Publications/wp-admin/includes/misc.php on line 557