How to recover lost GRUB bootloader in Linux

GRUB is a small program used to manage booting into multiple operating systems and specify boot options for each operating system. GRUB could get corrupted or deleted in events like installing Microsoft Windows, making you unable to load your other installed operating systems.

GRUB configuration is stored in other partition such as in your Ubuntu installation disk. It means if GRUB itself is corrupted or deleted, it can be re-installed and configured to read from existing configuration files.

1. Boot into Linux using Live CD / USB Drive.
2. Get into Live CD mode if available.

   For Ubuntu installer, click on the Try Ubuntu button.

3. Launch Terminal.

   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. Find Linux partition with working 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 temporary directory to mount Linux partition.

   $ mkdir tmp

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

   $ sudo mount /dev/sda1 tmp/

7. Mount /dev and /proc folders folders to the Linux partitions' mount point.

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

8. chroot to the Linux partition.

   $ sudo chroot tmp/
   root@ubuntu:/#

9. Re-install GRUB bootloader from 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 chroot environment.

    root@ubuntu:/# exit
    exit

11. Shutdown the system.

    $ sudo poweroff

12. Remove the Live-CD / USB Drive
13. Boot the system.