GRUB is a small program used to manage booting into operating systems, loaded by the MBR. It allows you to select operating systems to boot and specify boot options for each.
GRUB could get corrupted or deleted in events like installing Microsoft Windows, making you unable to load your other operating systems.
GRUB configuration is stored in your Linux root partition. It makes it possible to restore deleted or corrupted GRUB from the remaining configuration files.
Related: How to backup MBR in Linux
For Ubuntu installer, click on the Try Ubuntu button.
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).
$ 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
$ mkdir tmp
$ sudo mount /dev/sda1 tmp/
$ sudo mount --bind /dev tmp/dev $ sudo mount --bind /proc tmp/proc
$ sudo chroot tmp/ root@ubuntu:/#
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.
root@ubuntu:/# exit exit
$ sudo poweroff
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