Boot from Live CD / USB
open a terminal
sudo su
mount your linux partition (in my case /dev/sda5)
mount /dev/sda5 /mnt
mount specials
mount –bind /dev /mnt/dev
mount –bind /proc /mnt/proc
mount –bind /sys /mnt/sys
chroot to your mounted linux partition
chroot /mnt
update ad install grub
update-grub
exit from chrooted partition
exit
unmount all
umount /mnt/sys
umount /mnt/proc
umount /mnt/dev
umount /mnt
reboot