My dog just gave birth, supposedly 4 pups but the other one died.?

Literally my dog just gave birth to 4 pups, but sadly one pup died and we haven't noticed that our dog was giving birth. We actually did not know what the caused of the death, we saw the dead pup actually half bodied, what happened? What are the things to do for them? Planning to spay her after this. they looked healthy by the way, hope nothing bad happen with them and no pup will die again.