You will not be able to boot into your operating system if your MBR or Master Boot Record is corrupted or missing. If you were installing Windows, your drive would still have an MBR but it's configured to only boot into your Windows partition. In the instance that you have Linux with GRUB in other partitions; you'll not be able to boot into them unless you have a backup of your MBR or if you could reinstall GRUB using grub-install.

You will need to boot up from a secondary system such as Ubuntu installer disk if you are not able to boot into your Linux system. From there you can load up your MBR backup to be restored.

Steps to restore Master Boot Record from dd backup:

  1. Boot into Linux using Live CD / USB Drive if you can't boot into your system.
  2. Launch terminal.
  3. Check if your MBR backup is correct.
    $ file mbr.bak 
    mbr.bak: DOS executable (COM)
    $ cat mbr.bak 
    �c������t��pt���y|1��؎м ��d|<�t��R��t��}��|�A��U�ZRr=��U�u7��t21��[email protected]�D��D�f�\|f�f�`|f�\
    Z������}�f�ƈd�@f�D�������@�����f�f�`|f	�uNf�\|f1�f�4��1�f�t;}7����0�����Z�ƻp��1۸�r��`���1�������a�&Z|��}���}�4��}�.���GRUB GeomHard DiskRead Error
  4. List available disks and identify the one with the corrupted master boot record.
    $ lsblk
    loop0    7:0    0  55.4M  1 loop /snap/core18/1932
    loop1    7:1    0  55.4M  1 loop /snap/core18/1944
    loop2    7:2    0 217.9M  1 loop /snap/gnome-3-34-1804/60
    loop3    7:3    0   219M  1 loop /snap/gnome-3-34-1804/66
    loop4    7:4    0    51M  1 loop /snap/snap-store/498
    loop5    7:5    0  62.1M  1 loop /snap/gtk-common-themes/1506
    loop6    7:6    0    51M  1 loop /snap/snap-store/518
    loop7    7:7    0  64.8M  1 loop /snap/gtk-common-themes/1514
    loop8    7:8    0  31.1M  1 loop /snap/snapd/10492
    loop9    7:9    0  31.1M  1 loop /snap/snapd/10707
    sda      8:0    0    20G  0 disk 
    ├─sda1   8:1    0     1M  0 part 
    ├─sda2   8:2    0   513M  0 part /boot/efi
    └─sda3   8:3    0  19.5G  0 part /
    sr0     11:0    1  1024M  0 rom
  5. Restore the master boot record of the disk from the backup using dd.
    $ sudo dd if=mbr.bak of=/dev/sda
    [sudo] password for user: 
    0+1 records in
    0+1 records out
    446 bytes copied, 0.00284831 s, 157 kB/s

  6. Check if your MBR is correctly restored.
    $ sudo head -c 446 /dev/sda
    �c������t��pt���y|1��؎м ��d|<�t��R��t��}��|�A��U�ZRr=��U�u7��t21��[email protected]�D��D�f�\|f�f�`|f�\
    Z������}�f�ƈd�@f�D�������@�����f�f�`|f	�uNf�\|f1�f�4��1�f�t;}7����0�����Z�ƻp��1۸�r��`���1�������a�&Z|��}���}�4��}�.���GRUB GeomHard DiskRead Error
  7. Reboot your system to test.
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