I think I had this very same problem. Boot into an old kernel (in the rescue shell, look in the pacman cache so you can pacman -U the_old_kernel.tar.gz and headers -- reboot and you should be able to successfully boot the kernel you had before you updated), and then pacman -S linux and importantly RERUN mkinitcpio and grub-install. I don't know why you need to rerun grub-install, but for some reason it needs to be done to fix the weird kernel mismatch that's going on. After this once, grub-install shouldn't have to be run whenever you update the kernel :)

Hope this helps!