In order to update the grub configuration of the machine that you might be misconfigured and unable to boot up properly you could use live CD to boot into the machine, however, even if you are able to update the /etc/default/grub file, you will need to perform the "sudo update-grub" command to effect the change. if you are using Live CD to login, you need to perform the following steps:
First mount the root directory in question for my case it is my /dev/sda so it is it will be /dev/sda2 as mostly sda1 is for the /boot/efi partition.
You will mount the root partition as follow:
sudo mount /dev/sda2 /mnt
sudo mount --bind /dev /mnt/dev sudo mount --bind /sys /mnt/sys sudo mount --bind /proc /mnt/proc
How can you tell if you have a boot partition?
sudo chroot /mnt
sudo umount /mnt/dev sudo umount /mnt/sys sudo umount /mnt/proc sudo umount /mnt/