Are you on the unstable branch? I had exactly the same problem yesterday after an update from kernel. 126.96.36.199. to 5.2.02. I have always a second kernel installed. Booting with this older kernel, downgrading to the previous version of kernel 5 and reinstalling the kernel helped. Otherwise you need to use the method already suggested, i.e. to boot into a live system and restore the previous status using chroot. I usually switch temporarily to the stable branch in this situation, with
sudo pacman-mirrors -B stable --api
sudo pacman -Suuyy
However, this coincidence suggest that there is a more serious problem with the update procedure.