Restore the GRUB Bootloader
As with any Linux operating system, the GRUB (GRand Unified Bootloader) is responsible for booting up Manjaro. If for any reason your GRUB is not working - perhaps due to being corrupted, mis-configured, or even deleted - then it may not be necessary to reinstall Manjaro as a consequence. The GRUB can instead be fully repaired and restored, retaining your installed Manjaro system. To undertake this task, you will need to use your Manjaro installation media (e.g. CD/DVD or USB Flashdrive).
Boot the Manjaro Installation Media
1. Insert and boot your Manjaro installation media.
2. Select your preferred language (F2) and keyboard layout (F3).
3. Boot Manjaro Linux. It does not matter which boot option you choose, as the installation media is being used solely to repair/reinstate the GRUB, and not to install a fresh system.
4. Open the terminal or access the command line of the live CD.
- From the desktop environment: open the terminal from your desktop menu, or
- From the command line (e.g. NET-Edition or having used the Boot in Text mode option): enter the default username (manjaro) and password (manjaro) to log in.
Identify and Prepare the Installed Partition(s)
1. Ensure that you are using the Root account, which is identified by a hash ('#') at the beginning of the command line, rather than a dollar ('$'). To switch to Root, enter the command:
2. List your partitions. This is necessary in order to identify the partition your Manjaro system is installed on. If you have used a separate boot partition, it will be necessary to identify this as well. For a list of your paritions, enter the command:
In this particular instance, having used the assisted preparation method to install Manjaro earlier, the partitions for the author's Manjaro system are as follows:
- /dev/sda1: Boot partition
- /dev/sda2: Swap partition
- /dev/sda3: Manjaro system
- /dev/sda4: Space for personal files.
If you are still unsure, then you can also use an application called gparted, which should be selectable from your desktop menu. This will provide a simple visual illustration of the partitions on your hard drive(s). If a seperate partition for the GRUB exists, it will be marked as 'bootable', and should only be about 100MB in size.
3. Mount your Manjaro system partition. The syntax to mount the Manjaro system partition is:
mount /dev/[partition used for Manjaro system] /mnt
In this instance, as the Manjaro system partition is /dev/sda3, this will be mounted using the following command:
mount /dev/sda3 /mnt
IF you have used a seperate partition for your GRUB bootloader, then this must also be mounted. The syntax to mount a separate partition used to boot is:
mount /dev/[partition used for GRUB] /mnt/boot
In this instance, a seperate partition - /dev/sda1 - has been used for the GRUB, and will be mounted using the following command:
mount /dev/sda1 /mnt/boot
3. Change to the root directory of your mounted partitions. This is undertaken so that you are working from - and with - your installed system, rather than the installation media. To do so, it will be necessary to enter a series of commands in the following order:
mount -t proc proc /mnt/proc mount -t sysfs sys /mnt/sys mount -o bind /dev /mnt/dev mount -t devpts pts /mnt/dev/pts/ chroot /mnt
4. Install the software applications mtools and os-prober. mtools is a collection of tools to access Microsoft Operating Systems (i.e. MS-DOS). os-prober is a utility that detects if there are any other operating systems present. Install them with the following command:
pacman -S mtools os-prober
For BIOS Systems
1. Install a new GRUB bootloader with the following command:
2. Recheck to ensure the that installation has completed without any errors:
grub-install --recheck /dev/sda
3. Finally, configure the freshly installed GRUB bootloader:
All done! Now close the terminal and reboot your system to use your freshly re-installed GRUB.
For UEFI Systems
1.) First of all check the partition for the ESP (EFI System Partition). An ESP is a fat32 partition and contains .efi files for booting.
It can be checked using Gparted or from the terminal using fdisk -l
If you do not have a EFI partition, you will need to create it. Use Gparted for that.
Size- 512 mb to 1 gb
2.) Create the /boot/efi directory
sudo mkdir /boot/efi
3.) Mount the EFI partition as /boot/efi
sudo mount /dev/sdXY /boot/efi
X = Alphabet of the drive = a,b,c ... Y = Partition number of the EFI partition = 1,2,3,4...
Example - /dev/sda4
4.) Re-install Grub.
sudo grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=manjaro --recheck
5.) Update Grub configuration file.
See also UEFI Install Guide
Post for support at the forums- 
Arch Linux is not recognized
If Arch Linux wasn't recognized after an update-grub then probably your Arch installation is missing the package lsb-release.