I had a silkie chick that had that happen, it just dried up and later fell off sometime during the night. I kinda carried it around everywhere though instead of leaving the chick alone as it was the first one to hatch and kept crying for attention every time I put it down. 
I thought it was a yolk sac at first, but it is the umbilical cord attached to the poop sac that is usually found on the pointy end of the egg.

I thought it was a yolk sac at first, but it is the umbilical cord attached to the poop sac that is usually found on the pointy end of the egg.

