This happened to me recently when the update process was interrupted (gnome-shell froze part-way through the update and I killed it, but hooks didn’t run even though pacman is supposed to survive parent processes being killed, … or something else odd happened).
You’ll need to boot a live installer environment (or other Linux boot disk) and chroot into your installation, then:
pacman -Syu linux-zen
mkinitcpio -p linux-zen
update-grub
This should restore linux-zen to operation - adjust that for any other kernels you have installed.