chroot grub repair

grub2-mkconfig -o /boot/grub/grub.cfg


This must be run first:

mount /dev/sda1 /mnt
mount --bind /dev /mnt/dev
mount --bind /dev/pts /mnt/dev/pts
mount --bind /proc /mnt/proc
mount --bind /sys /mnt/sys
Plus, if booted off a flash recovery drive,

chroot /mnt