Is she drinking and acting normally? If so and it is just one meal don't worry about it. I have one who does that from time to time. If she hasn't eaten for more than 24 hrs, go to the vet. If she has stopped drinking as well definitely get her to the vet,
For that question, you have to know your pet. If she is tired of the food she normally eats, buy a small bag of something new and try it. Or ask your local vet what is the best type op food for her breed(unless she is a mix), age, and activity level(Is she more an indoor pet?). Also, observe her; if she acts listless, doesn't want to do much, she may be sick. If you suspect that, take her to a vet pronto. I hope this helps, and that she feels right back her puppy self again!
I have found that my dogs will munch down a plate of scrambled eggs like nobody's business most of the time, even if they don't want their regular stuff. If she usually eats dry food, check her mouth to make sure her teeth/gums aren't infected and sore. Make sure she's getting fluids, you don't want her to get dehydrated.
hmm.well how old is she?if shes a puppy and you give your non puppy food then there can be something wrong.or if shes a grown up and you give her the same one ever.since she was a puppy then change her food.if its nothing that then take her to the vet.or change her food and see.if she is still like that or not.