Um, are you sure that's a sack out of it's vent? To me that looks like unabsorbed yolk sack connected to its umbilical cord hole (not sure what you call that hole). Are there any internal organs outside the chick's body? If so, you will want to push those back inside if possible. The umbilical cord hole closes up within a few hours of hatching - if the internal organs are not back inside the chick before the hole closes up, you will have to cull the chick. I had a chick with this problem and was unable to push everything back in. I had to cull it. I'm still sad about it, but there was nothing else I could do.
If this is unabsorbed yolk sack, set the chick in a small cup so it won't move around much, inside a soft paper towel, and give it a few hours inside the incubator to see if it will absorb the yolk. Details in this article:
https://www.backyardchickens.com/articles/guide-to-assisted-hatching-for-all-poultry.72886/
If there are internal organs outside its body as well as yolk sack, and you can't get them back inside, you will need to cull the chick. You can cull it quickly (cutting board and knife or a strong pair of scissors) or it will die a lingering painful death over the next few days as its organs slowly die. Just not much you can do when its insides are on its outsides. Surgery for a chick this small is out of reach for most of us backyard chicken keepers.
I really hope only the yolk sack is out and that it absorbs quickly.