Did you by accident uninstalled your kernel?
I would suggest booting up a live iso of Manjaro and then chrooting into your broken installation (it will give you shell to fix stuff ) and try installing linux56 or better yet something more recent like linux-latest (or something similarly called).
Chrooting is not as difficult as some people claim.
After booting live iso connect to internet then you open up terminal and attempt
sudo manjaro-chroot -a
and then install
sudo pacman -Sy linux-latest
If it fails with can’t find mount point 0! then you used something like full disk encryption and you better consult here for further guidence if wikipage article is not helping you.