I am so very sorry. It is always desperately hard to lose a much loved dog, and not knowing the cause can make it harder.
Poppy, my toy poodle, was diagnosed with acute liver failure just over a year ago. The signs were not obvious - she had a bout of bloody diarrhoea which cleared up quickly with metronidazole, but a week or so later started vomiting and then, for the first time in her life, refused a meal. Her vet did blood tests, although nothing was obviously wrong on examination, and phoned me to tell me to come back immediately for antibiotics - the liver results were off the scale. By next day she was very ill, and she spent two days at the vets on IV fluids and medication, coming home overnight. The plan was to do a biopsy to identify the exact cause, but she was too ill, so was treated with steroids, Destolit to thin bile, and Denamarin. She was more fortunate than your poor dog, and has recovered enough to have a good quality of life, despite the disease now being chronic.
My understanding is that many liver issues require a biopsy for a firm diagnosis - sometimes cancer is obvious from a scan, but not always. Blood tests will show that the liver is failing, but not necessarily what is causing the failure - with Poppy the hypothesis is that it was an infection, possibly related to pancreatitis, but we will probably never know. As I am sure you have, I have wondered whether she could gave picked up something toxic, or whether it could have been diet-related, or something else I could have prevented or might be able to prevent in the future, but have not been able to think of anything. I know that there have recently been dog foods recalled in the US for high aflatoxin levels, and that aflatoxin can cause liver disease, but then so can so many other things.
Grief hurts, and you must still be in shock, and trying to make sense of your loss. If it would help a little later to tell us your poodle's name and more about him there is a Pet Memorials section in the forum - many of us know the pain of losing a dog suddenly and too young, and can sympathise with how you are feeling.
