What do you do if your linux kernel upgrade did not go well? What if after an upgrade which includes the kernel, you can’t boot your system?
You should downgrade to an older, working linux kernel still in the cache. Using a fairly recent Arch linux install medium, you can mount the root partition to /mnt. Then chroot into the system.
$ arch-chroot /mnt /bin/bash
It is assumed you keep at least one other version of the linux kernel in the cache. You can check in /var/cache/pacman/pkg. At least downgrade linux, linux-header and any kernel module you have. To downgrade:
$ pacman -U linux-x.xx.xx-x86_64.pkg.tar.xz linux-headers-x.xx.xx-x86_64.pkg.tar.xz
Exit by typing exit. Reboot. You should be able to boot now.