after run pacman -syu and reboot Gnome 3 can not start

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https://bbs.archlinux.org – yesterday I run this command pacman -syu and my system start download over 500 MB and I go to shop , after I come back , I see everything downloaded and I shutdown my system and go to my friend home . after come back I turn on my system and I see my system start , but Gnome3 can not start and my system go to command line and can not go to graphical mode and I can not enter my pass and username . before this I use GDM for login . last Gnome was Gnome3.4 but new Gnome is 3.6 , I think GDM has big problem and can not start , how I can solve this problem ? mfaridi https://bbs.archlinux.or (HowTos)