If your Bichon has not eaten in 5 days take him or her to a vet as an emergency. A Bichon is never quiet unless they are in pain. Five days and lethargic. Something is very wrong. Find an emergency hospital. He may have eaten something or he may have a tick bite. Kidney stone? Is there any signs of foul play/ Neighnors garbage? check his urine for blood. Use a sryinge to give him water. My mom was a breeder and nor to alarm you but the dog is telling you something is really wrong.
Be prepared blood work and x-rays. .I would not just take the dog to any clinic. Make sure they have the facilities and equipment to put it all together.
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Good luck
AnnieSEZ