Just running ‘update’ again wont necessarily fix anything.
This is the basic guide.
Which I thought provided some more steps than that … but it seems the only addition is ‘update-grub’, which may or may not even be present on newer systems.
It would also make sense to rebuild initramfs, so I will rehash the steps after chroot here;
pacman-mirrors -f
pacman -Syu
mkinitcpio -P
grub-mkconfig -o /boot/grub/grub.cfg
The final line is equal to ‘update-grub’.
I might suggest also checking /var/log/pacman.log to see where/when the interruption actually occurred.
(becuase you may have to cleanly reinstall certain packages)