This happened to me during my first hatch, so I know how scary it is! My 2nd chick hatched day 21, with no assistance with what appeared to be a full unabsorbed yolk sac attached. Thankfully, there was no intestine involvement. We quickly taped some cardboard to the inside of the incubator to quarantine the chick in the corner it hatched. We then did some frantic searching for what to do. There are quite a lot of posts regarding this on this site. We determined there wasn’t much more we could do, except hope it absorbed the yolk overnight.
The next morning the sac was dried, but there was yolk smeared everywhere. The chick was half the size of the other chicks, and because we were unsure how much yolk it absorbed we fed the chick with a syringe. Thankfully there was a vent hole in the quarantined corner, so we could feed it without opening the incubator while the other chicks hatched.
Each day we went to sleep with our fingers crossed it would be alive the next day. After 5 days keeping it in the incubator, we washed it. It had too much smeared muck on it to fluff up. We also trimmed the dried sac, because it hadnt fallen off. Once fluffy we trimmed the umbilical cord/sac again, so it was disguised by the fluff. We then introduced it to the other chicks. It was tiny, but fit in with the others with no problem.
The chick is now a month old, the same size as the others. In fact you couldn’t tell it had any trouble in its early days. From what I’ve read we were lucky, because it could just as easily gone the other way.
Hopefully, this gives you some hope that your chick will be ok too. Good luck.