If she's got a simple upset digestive system, she won't want to eat. But it should settle in a few days. Dogs can go much longer without food than they can without water.

If she's not drinking, she needs to see a vet asap! Dehydration can get dangerous very quickly. I almost lost Preacher once to dehydration. After only a couple days of not drinking, his kidneys and liver were shutting down from dehydration. How much less is she taking in than she usually does? Is she vomitting too? Is she lethargic?