Recover partition table with 'testdisk'.

fsck all partitions, be sure to check lvm partitions.

Choose to mount under /mnt/sysimage

chroot /mnt/sysimage

grub-install /dev/sda

If you get this problem: /dev/sdb1 does not have any corresponding BIOS drive, edit the /boot/grub/device.map temporary to point to /dev/sdb. Reinstall grub and change back the device.map file.

exit

reboot