How to recover lost GRUB bootloader in Linux

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

Boot information is still available in Linux's installation disk and GRUB could be re-created from the available boot information.

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/
   [email protected]:/#

9. Re-install GRUB bootloader from existing configuration files.

   [email protected]:/# 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.

    [email protected]:/# exit
    exit

11. Shutdown the system.

    $ sudo poweroff

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