I would xray the bitch and count the pups, or maybe ultra-sound the bitch to see if it can be determined if the pups are alive. I would also do a swab/culture of the uterus.

The first thing I would suspect would be acute metritus (a massive uterine infection), and I would do this ASAP as it can be fatal very quickly.

If not that, I would be stumped at this point.

Jack