I had:
/boot unencrypted on /dev/sdb5
/ encrypted on /dev/sdb6
Grub screwed up and it was not obvious how to restore it, pointing it to an encrypted volume. Here is how I rescued the system. I post it as a reference if somebody has the same problem.
# Run the livedvd distro
# If encrypted volume was not opened (next 4 lines):
lsblk --fs # List Luks devices
cryptsetup luksClose root_fs # root_fs is the encrypted partition name
mkdir /mnt/system
cryptsetup luksOpen /dev/sdb6 root_fs
ls -la /dev/mapper/root_fs # root_fs is the encrypted partition name and should appear here
# If you don't have lvm2 on your livecd (next 5 lines):
apt-get install lvm2
vgscan
vgchange -ay
lvscan
ls /dev/mapper/ # See the situation
# If encrypted volume is open
mount /dev/mapper/root_fs /mnt/system/
#mount /dev/mapper/root_fs-home /mnt/system/home/
mount /dev/sdb5 /mnt/system/boot/
mount -o bind /dev/ /mnt/system/dev/
mount -o bind /proc/ /mnt/system/proc/
mount -o bind /sys /mnt/system/sys/
chroot /mnt/system
grub-install /dev/sda
update-grub