Upgrade problem: Notice: unserialize(): Error at offset 0 of 18 bytes in /home2/zooexit/public_html/kaoolo.com/index.php

Hi guys. I need some help. I tried to upgrade to the latest 2.3 version. Big problem. please click on this link http://www.kaoolo.com to take a look a the error message when I lunch the site. I can't even access the admin anymore.

please help