Are all mirrors in /etc/pacman.d/mirrorlist still up-to-date? Assumed the installed version of the package pacman-mirrorlist should be up-to-date, check if the mirrors are still available by /etc/pacman.d/mirrorlist.pacnew. Since your official _un_supported tool yaourt failed, you at least need to run, pacman -Syu and not just pacman -S, even if the mirror list still should be ok.
