If you're able to boot to another Ubuntu system, you can chroot to your root partition.

Check which partition you're using for your broken installation. I'll use /dev/sdb3 in this example. Replace the value of the root variable below:

root=/dev/sdb3

cd /

sudo mkdir /mnt/repair

sudo mount "$root" /mnt/repair

sudo mount -t proc proc /mnt/repair/proc

sudo mount -t sysfs sys /mnt/repair/sys

sudo mount -o bind /dev /mnt/repair/dev

You may need to mount /boot and /boot/efi. Simply use sudo mount /dev/sdb2 /mnt/repair/boot where /dev/sdb2 is the boot partition.

Now, to chroot, use this:

sudo chroot /mnt/repair

You're now logged in as root in your broken Ubuntu installation.
You can now do anything you want to repair your system.

Once you're done, use the following commands to clean up:

# Make sure there are no processes running in your chroot session

# Then, exit

exit

sudo umount /mnt/repair/{proc,sys,dev}

sudo umount /mnt/repair

Reference