Welcome to the forum!
Do you mean that you’ve installed and used
grub-customizer? In that case, that’s what hosed your GRUB installation.
grub-customizer is in the repositories because we follow Arch, and Arch also has that package in its repositories, but the truth of the matter is that more often than not,
grub-customizer completely ruins the boot loader configuration, especially given that Manjaro uses a heavily customized version of GRUB, with which
grub-customizer is not compatible.
Here’s what you can do…
Boot up from the live medium from which you installed Manjaro.
Open up a terminal window, and in this terminal window, type the following command…
- Enter the root password, and then issue the following command…
rm -f /etc/grub.d/*
pacman-mirrors -f 5 && pacman -Syyu grub
grub-install --no-rs-codes --recheck --modules="part_gpt part_msdos"
sync && exit
Unless there are any additional problems that you haven’t told us about yet, you should now be able to reboot into your installed system. And please, please, please, don’t ever use
grub-customizer again. In fact, I recommend that you uninstall it.