Oh no!
I am so so sorry. That is so sad. With a big heavy mommy like that... aw poor little puppy.
My Rottie had a litter when I was a teenager, and one night soon after the babies were born I found one of them cold and lifeless. It is so heartbreaking. I know exactly how you feel.
Is everyone in a "whelping box"? I have seen a design that has these ledges on the sides that are supposed to help prevent mommies laying on babies... here is a picture of one, it has the railings around near the bottom. Maybe if yours doesn't have those you could put some in??
I sure hope the rest of the babies will stay safe. Hang in there. Now there will be more milk and love for the ones you still have. Again I am so sorry.