Recently I was installing Linux Mint 19 onto a computer and discovered that the installer consistently failed to setup GRUB correctly. This left the system in an unbootable state. Thankfully there was an easy work around!
From within the live CD environment, simply quit the installer once the installation has completed (and grub has failed). Then install the Boot-Repair utility:
sudo add-apt-repository ppa:yannubuntu/boot-repairâ€© sudo apt-get updateâ€© sudo apt install boot-repairâ€© sudo boot-repair
Once Boot-Repair has loaded click the arrow next to Advanced options, then navigate to the GRUB Location tab. On this screen simply press Apply and allow it to do the work for you. After the bootloader has been fixed you can restart your computer as normal and boot into your new OS!