As you used a hen which is the most natural way to incubate I would say there could have possible been something wrong with egg quality or the parent birds of the chicks.
Good luck with your hen and her hatched chicks and enjoy BYC
I am so sorry you had some trouble here. Was this a broody hatch? So many things could have gone wrong. It is hard to say what the cause of the poor hatch. Feel free to follow the link to our Incubation forums or even stop by our Chicken Behavior and Egglaying section too where lots of broody talk goes on... https://www.backyardchickens.com/f/18/chicken-behaviors-and-egglaying
Good luck in the future with your hatches and welcome to our flock!