There are times we lose our Manjaro boot and we cannot boot back into Manjaro.
The usual causes are
o when we install another linux distro
o when another linux distro gets updated
o when we reinstall Windows
The best and easiest way to handle this problem is to have a grub rescue cd handy. In this case, use the grub rescue cd to boot into Manjaro and proceed straight to "When booted" section below.
If in the rare case this is not available, here are some steps to get back Manjaro.
And all the following applies only to bios-legacy/msdos.
If system is uefi/gpt, just go to uefi bios to reset Manjaro as default boot and then "update-grub" if necessary
(Case 1) Another linux bootloader has become the default
and the problem is usually due to the other OS picking up just only Manjaro's intel-ucode.img as its initrd file.
Use that other linux bootloader to boot into Manjaro
There are several ways to do this, but I will mention only one.
At that other linux grub menu, go into its grub prompt (grub>) by pressing 'c'
At its grub prompt,
grub> search.file /boot/intel-ucode.img root
grub> configfile /boot/grub/grub.cfg
Manjaro grub menu will then appear, enter at the first (top) entry and we will boot into Manjaro.
Jump below to the "When booted" section below.
(Case 2) Windows installation.
Boot up a livecd, at livecd terminal,
sudo mount /dev/sdxy /mnt
sudo grub-install --boot-directory=/mnt/boot /dev/sda
where /dev/sdxy is Manjaro partition -- check with "lsblk -f", "sudo blkid", "sudo parted -l" etc to be sure to get partition correct.
When booted into Manjaro, at terminal,
sudo grub-install /dev/sda
Reference notes -
manjaro forum link
grub manual -external
If we choose to use the other linux bootloader as default boot, make a corrected entry for Manjaro and put it in the other OS /boot/grub/custom.cfg. See here.