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Boot up from the Manjaro USB drive (or optical medium) in live mode. When up and running, open up a terminal window and type… 
sudo su-
Enter the password, and then enter the following command…
manjaro-chroot -a
If you have only one GNU/Linux installation on your system, press 1. Then enter the following commands ─ it includes checking for updates…
pacman-mirrors -f 5 && pacman -Syyu
grub-install --recheck --modules="part_gpt part_msdos"
update-grub
sync && exit
It should now be safe to reboot.