Animal experts?

[Not Afraid]

Yes, take your dog to the Vet, today if possible.

Does the dog smell ok? I'm thinking a scent of some sort may have made it pass out. Skunk perhaps (although you'd probably know that one). Maybe a large wild cat sprayed your dog? Do you have cougars, pumas, etc in your area? That would explain the size of the beast. How old is your dog? Maybe a small heart attack or stroke?

I'm just guessing here, a Vet could tell you more.