Using grub rescue to boot machine and repair MBR

Use “ls” to find your partition list:
ls
(hd0) (hd0,msdos3) (hd0,msdos2) (hd0,msdos1)

Check the content of each to find your Linux partition:
ls (hd0,msdos3)/
ls (hd0,msdos2)/
ls (hd0,msdos1)/

You want the one with the /boot folder. In our case, this is (hd0,msdos3). The following commands will boot your Linux OS.

set prefix=(hd0,msdos3)/boot/grub2
set root=(hd0,msdos3)
insmod normal
normal

<root password for maintenance>

Use “df” or “mount” to figure out which disk holds the Linux partition. In our case, it is /dev/sdb. You don’t want the partition (e.g. /dev/sdb2) but the disk. The following commands install the grub2 bootloader and build a config file.

grub2-install /dev/sdb
grub2-mkconfig -o /boot/grub2/grub.cfg