Wow, I had this issue this very evening… booted from USB, chrooted, then searched for available kernels.
I figured I’d have a go at adding and removing kernels like this:
pacman -S linux59 && mkinitcpio -P && pacman -R linux54 && mkinitcpio -P && pacman -S linux54 && mkinitcpio -P && pacman -R linux59 && mkinitcpio -P
Then update-grub then reboot. Selected 5.4 LTS kernel on reboot.
I’m back, did you miss me? 