My Macbook won't start after a automatic update from "Updater"

Hey all,

I just tried to update my iPhone with iTunes but it said I was need to update my iTunes to 10.5, I updated it and I got a popup which shows the Mac auto-updater; it found 900 mb of updates. I updated and it ask for reboot. I reboot but now the loading screen of my Macbook is stuck and it wont start...

plz help 😀