These are the directories to mount:
# mount -o bind /proc /a/proc
# mount -o bind /dev /a/dev
# mount -o bind /sys /a/sys
# mount -o bind /run /a/run
# chroot /a /bin/bash
Once in, you may update your grub, but please check /etc/fstab before…
# grub2-install /dev/sda;grub2-mkconfig -o /boot/grub2/grub.cfg