I don't know what happened. Her failure to "wake up" sounds like a combination of shock and medications causing cardiac arrest.
The size of pups and number could have caused pressure on the descending vessels in the body (caudal vena cava/descending aorta) when anesthesia happens and the abdomen is opened and the uterus is manipulated and pressue is removed it is like unkinking a hose and the vessels fill again and cause all kinds of problems. Unlike human hospitals, we don't have the sophistication of the monitoring devices to see problems as quick as they happen...and truthfully the vet is focused on getting the pups out...there is a staff member to take each puppy and work on them while the vet trys to sew up the uterus or perform the spay.
Since she felt the dog was in shock and need blood vs. fluids partially into the surgery...that leads me to believe that there was excessive bleeding.
I don't know your vet, but let me say that the majority of us are in this profession to help and treat animals because we care so much for them. We tend to be very upset when we lose one, regardless of the cause. If I was in a similar situation, I would want to discuss what happened with you..but I don't know if I would be calling you at home to see how your were unless we had a relationship.
I guess my recommendation, as you process the massive loss you have suffered...is to call the vet office and ask for an appointment with that vet. Go in and spend time asking all these questions...ask about the health of the uterus when she went in, where the blood loss was coming from, etc.
As an RN..you know the unexpected happens, but it does help to hear the words from your vet...she may be waiting for you to be ready to talk about it.
I hope this helps. I am truly sorry for your loss.