If your pup won't eat for 24 hours you need to see the vet or sooner if their are any other symptoms like diarrhea, vomiting, lethargy or depression, fever, or a change in drinking too.
When I brought my Shih-Tzu home she was only 1 pound. No room to loose weight. Go get baby cereal and just like a baby feed it a little every couple of hours with a dropper, warm it just a little. Also so have your vet check for worms or any problems. The cereal won't hurt it, actually they love it it will put some weight on them. You might also think about when this started and figure out if the dog is just stressed. Believe it or not they get stressed and God only knows why. I hope everything turns out well. I will keep you in my prayers.