You could try to repair the update in terminal in a live session. Mount your root and do a arch-chroot:
mount /dev/sdXY /mnt
arch-chroot /mnt
where XY is your root partition. If you have additional partitions (e.g. separate boot or an efi partition) mount them accordingly before the arch-chroot:
mkdir -p /mnt/boot/efi
mount /dev/sdXZ /mnt/boot
mount /dev/sdXA /mnt/boot/efi
Then check if the update was ok by
pacman -Syyu
If this was ok then do a
mkinitcpio -p linux
and
grub-mkconfig -o /boot/grub/grub.cfg
Then cross fingers and reboot…