My dog suddenly vomited yesterday, with drooling, no appetite and low energy.

There are many reasons a dog may vomit and act listless, and your information is incomplete, so here are the common ones. First, gastritis or pancreatitis, often from irregular feeding or greasy, hard-to-digest food. Second, intestinal parasite infection such as tapeworms or coccidia, which disrupts gut function and can cause diarrhea. Third, viral infection such as parvovirus or coronavirus, which starts with digestive symptoms and can lead to dehydration, wasting and death if ignored. Solutions: take the dog to a vet to identify the cause; if a vet is impossible right now, withhold food and water for half a day. Once vomiting stops, give cimetidine tablets to stop vomiting and amoxicillin powder to reduce gut inflammation, plus easily digested food and probiotics. Keep a regular diet, feed small frequent meals, deworm regularly, and keep vaccinations current.