After booting into the live Ubuntu environment, open a terminal from the Dash and run the following commands to install Boot Repair:
sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt-get update
sudo apt-get install -y boot-repair
The Boot Repair window will appear after you run the boot-repair command. After it scans your system, click the Recommended repair button to repair GRUB2 with a single click.