Restore GRUB
Mount the linux partition, chroot to it, run grub-update. Or: fdisk -l list of physical devices (X) and partitions (Y) (or blkid) mount /dev/sdXY /mnt umount /mnt/boot and mount /dev/sdXY /mnt/boot if you have a separate boot partition grub-install --boot-directory=/mnt/boot /dev/sdX (Given you have access to a busybox/linux shell)