For accessing a problematic system
[HowTo] Resolve an unbootable Manjaro (black screen, login failed, cant boot, etc)
Mirrors and update
sudo pacman-mirrors -f && sudo pacman -Syu
mhwd-kernel to install kernel
sudo mhwd-kernel -i linux66
Alternatively if the above does not work, installing kernels through pacman:
sudo pacman -Syu linux66
Rebuild initram and update grub
sudo mkinitcpio -P
sudo grub-mkconfig -o /boot/grub/grub.cfg