Mod-edit: Formatting fix for commands.
Simple for beginners:
your last update has not finished completely.
.
Start from the live ISO. Select your language and your keyboard-config. Free Drivers is o.K.
Wait until Desktop has finished loading.
Now open a Terminal, thats in icons a black rectangle…
Then enter the command:
sudo manjaro-chroot -a
Asks for Password, that is: manjaro
This means, the software on your ISO knows, that you want to change to your main-System,
which is corrupt at this time being…
In this terminal enter:
sudo pacman -Syyu
That means: continue the broken update of your main-System
Thereafter:
sudo update-grub
That means: repair the bootloader (if necessary) of your hopefully EX-broken System.
The reboot is required to finish all actions done.