How to recover lost GRUB bootloader in Linux

The GRUB bootloader is a small, essential program that manages the boot process for multiple operating systems on your computer.

It can be corrupted or deleted during events like a Windows installation, preventing access to your other operating systems. Fortunately, you can restore the GRUB bootloader using the remaining configuration files located in your Linux root partition.

1. Boot your computer using a Linux Live CD or USB drive.
2. Get into Live CD mode if available.

   For Ubuntu installer, click on the Try Ubuntu button.

3. Open terminal application.

   If no Live CD mode available, press <ctrl> + <alt> + <f2> keys to get to the terminal. For Ubuntu, the default username is ubuntu and the password is empty (just press enter at the password prompt).

4. Identify the Linux partition containing the functional GRUB configuration.

   $ lsblk
   NAME   MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
   loop0    7:0    0  1.9G  1 loop /rofs
   loop1    7:1    0 89.3M  1 loop /snap/core/6673
   loop2    7:2    0 53.7M  1 loop /snap/core18/941
   sda      8:0    0   20G  0 disk
   └─sda1   8:1    0   20G  0 part
   sr0     11:0    1    2G  0 rom  /cdrom

5. Create a temporary directory to mount the Linux partition.

   $ mkdir tmp

6. Mount the Linux partition to the temporary directory you created.

   $ sudo mount /dev/sda1 tmp/

7. Mount the /dev and /proc folders to the mount point of the Linux partition.

   $ sudo mount --bind /dev tmp/dev
   $ sudo mount --bind /proc tmp/proc

8. Use the chroot command to change the root directory to the Linux partition.

   $ sudo chroot tmp/
   root@ubuntu:/#

9. Reinstall the GRUB bootloader using the existing configuration files.

   root@ubuntu:/# grub-install /dev/sda
   Installing for i386-pc platform.
   Unknown device "/dev/sda1": No such device
   Unknown device "/dev/sda1": No such device
   Unknown device "/dev/sda1": No such device
   Unknown device "/dev/sda1": No such device
   Unknown device "/dev/sda1": No such device
   Unknown device "/dev/sda1": No such device
   Installation finished. No error reported.

10. Exit the chroot environment.

    root@ubuntu:/# exit
    exit

11. Shut down the computer.

    $ sudo poweroff

12. Remove the Live CD or USB drive.
13. Boot the system.