Google yolk sac infection, that should explain it. What I read is that it's more common in artificially incubated chicks. I've also noticed that it's more likely to occur in chicks that don't hatch on their own. You helped this one, right?
I am new to this; first chick hatched last night. Found it dead this morning. Is there anything I can do after the chick has hatched? Hen is sitting on the eggs, and a second one is starting to hatch, I hate to lose any more. Any advice will be much appreciated.