My dog foamed, then vomited yellow fluid, and has not eaten or drunk for two days – what’s wrong?

Based on your description, vomiting yellow fluid with no eating or drinking and lethargy may be caused by canine distemper, parvovirus, acute gastritis or pancreatitis. If the dog is regularly vaccinated, infectious disease can be ruled out. You can try symptomatic care with pet anti-emetic and probiotic products and watch for improvement. If there is no improvement, take it to a veterinary clinic promptly.