Only way to tell would be to get a vet to do a post mortem. There are endless causes of death which do not show up on the outside.
Most likely if not vaccinated would be pasteurella or a clostridial disease but could be bacterial infection, worms, coccidia, fluke, twisted gut, mineral deficiency, bloat or a host of other conditions.
Best bet would be get a faecal sample from the others to the vet to send off to check for worms, coccidia and fluke, if the sheep is less than 24 hrs dead get a post mortem done, and get them vaccinated asap.