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Try this one, first from USB/Live CD (if you can't access Ubuntu from HDD), and then, or directly from your "HDD OS":

sudo fdisk -l

This will list all partitions on your HDD. Find under the Type section your root partition labelled Ubuntu or Linux and its Device (like /dev/sda).

Then, do

sudo grub-install /dev/sdX

Where sdX is the device you found with fdisk. Note that you install GRUB to a drive, not a partition, so sda would be correct while sda1 would be incorrect.

Finally, run

sudo update-grub
sudo reboot

I also got this when trying to update from 16.04 LTS to 18.04 LTS. However, when trying the sudo grub-install command as mentioned I get:

Installing for i386-pc platform.
grub-install: error: failed to get canonical path of '/cow'.

But by following these instructions instead I solved my problem:

How to Repair, Restore, or Reinstall Grub 2 with a Ubuntu Live CD or USB