Shell

One of the solutions would be to mount an USB stick with Ubuntu on it.

After Ubuntu Live is started, boot-repair-disk should be installed.

*How to install:

sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt-get update
sudo apt-get install -y boot-repair && boot-repair

Run it through dash or through terminal.

After running "recommended repair", most of the problems regarding GRUB should be fixed.