In case when you install Windows when Linux is already installed, Windows overrides Linux GRUB with it's own bootloader without support of two operational systems. Here is algorithm how restore Linux GRUB. Lets pretend Linux has partition /dev/sda6
sudo mount /dev/sda6 /mnt sudo mount --bind /dev /mnt/dev sudo mount --bind /proc /mnt/proc sudo mount --bind /sys /mnt/sys sudo chroot /mnt grub-install --recheck --no-floppy /dev/sda exit sudo umount /mnt/dev sudo umount /mnt/proc sudo umount /mnt/sys sudo umount /mnt reboot
