Difficulty: ★★☆☆☆
So you couldn’t wait until the update process had completed and you went for the reset/power button, didn’t you? 
And now you’re getting the message that you don’t have a kernel installed. Well, you’re not the only one — we’re seeing it happen all the time. 
So, here’s how to fix it, you impatient n00b. 
If you don’t happen to have a recent Manjaro installer USB/DVD/CD handy, direct your browser to this wonderful page, download the .iso of your choice, put it on a USB stick in the proper manner — Ventoy is a great way of creating bootable USB sticks, and it’s available for Microsoft Windows too if that is what you’re currently using — and then boot up from it in live mode.
Once the desktop has loaded, open up a terminal window. In this window, issue the following commands… 
sudo su -
manjaro-chroot -a
If you have more than one GNU/Linux system on your machine, select the number that corresponds to the Manjaro installation that needs to be repaired. Then proceed with the following commands…
[ -f /var/lib/pacman/db.lck ] && rm -f /var/lib/pacman/db.lck
pacman-mirrors -f
pacman -Syyu
update-grub
exit
Be sure to let all of the above commands take their time and finish up. When prompted for the replacement of certain packages, answer Y.
After all of the above, cleanly reboot your machine and remove the USB stick.
Useful sites…