We are experiencing the same thing, one minute he is alert following us around the house, the next, he is completely out of it. He will not eat dog
food at all, so we feed him whatever he will eat (in small portions) and pray he keeps it down. We cheered when he had a bowl movement on Sunday. He kept down dinner last night, so we are hopeful. This seemed so sudden, even my vet was shocked, as she had seen him 1 mo prior and he was great! I am curious, what type of food was your dog eating?