IIRC it is normal that (most) summons die when you sleep.
If you load an old save can you heal summons again? I think you should be able to cast Aura of Guarding and Spell Shield on them, as well.
Can you bump the guardian off from following you if you cast Aura of Command on multiple summons or resurrected opponents?
Can you damage it with Poison Cloud, or Hell Spikes, etc? I don't recall if they affect allied creatures.