
I'm so sorry, I can imagine how upset you must be.
A similar thing happened with a friend whose bitch was in whelp to one of our dogs; she had a gut instinct at midnight that the bitch should have produced after straining for some time - the vet on call told her to wait until the morning, when she produced a dead puppy. Hoever, with her, she did have more live pups to follow, but it doesn't instill confidence, does it?
The only thing I can suggest is to make sure that if/when you mate her again, you have a good person to scan her, so that you can be prepared for a singleton, and if necessary for a caesarian before you lose the pup.
Anyhow, hope all goes well for you next time - feel for you now
Jo and the Casblaidd Flatcoats