You need to reinstall grub to your boot partition, which can be done as follows:
boot to a linux live system (e.g. Ubuntu Desktop).
assuming your actual boot partition is /dev/sda1 run the following:
sudo apt-get update && apt-get install grub2
sudo mount /dev/sda1 /mnt
sudo grub-install --root-directory=/mnt/ /dev/sda
restart your system, do not forget to remove the live image.