I was fed up of Windows update erasing my grub entries. So I have documented the steps to fix it, the steps which works for me. I hope it helps me when I lost my grub bootloader again.

To fix the bootloader, follow the below steps:

1. Boot from a Manjaro Live USB
2. Open Terminal
3. Type below command:
   

sudo manjaro-chroot -a

grub-install /dev/sda

grub-install --recheck /dev/sda

update-grub

Source: https://gist.github.com/dianjuar/a6e0d801a9f71f8edc06321178390320