feed them Baby Brine Shrimp, grinded up flakes that you feed your adult guppys. Just grind them up if your fingers. Make sure they are small small small pieces or you can feed them small parts of the yolk from a hard boiled egg! 2-8 times daily
its possible that your guppy already gave birth to all of her fry but they were eaten by the other fish. i would put some plants in there for the fry to help them survive and hide.