grub2 repair

1. boot rescue

2. mount root (and boot) partition

3.

mount --bind /proc /mnt/sda2/proc
mount --bind /sys /mnt/sda2/sys
mount --bind /dev /mnt/sda2/dev

4.

chroot /mnt

5. Then, inside 'chroot'ed  environment (not necessary if grub.cfg exists)

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

6.

grub2-install /dev/sda

(resp. grub2-install --force --debug /dev/sda )

7. exit, reboot