This is the commands needed to reinstall grub, that or checkout the Proxmox page
mount /dev/md2 /mnt
mount /dev/sda1 /mnt/boot/efi
mount --bind /dev /mnt/dev
mount --bind /proc /mnt/proc
mount --bind /sys /mnt/sys
chroot /mnt
grub-install /dev/sda
grub-install /dev/sdb