rkochar

• Boot into Manjaro on usb stick
• ctrl+alt+t
• sudo manjaro-chroot -a
• grub-install <partition> (eg: /dev/nvme0n1p6 or /dev/sda)
• grub-install --recheck <partition>
• update-grub
• exit
• reboot (remove usb stick, grub should re-appear with Manjaro as a boot option)

Grub may not add Windows (or another boot option) to the menu. Boot into Manjaro.