I have had that happen a bunch this year. Some made it out on their own, some didn't. I know a lot of people say to leave it alone, but I won't. I help it zip the rest of the way, VERY slowly, and then it kicks it's way out of the membrane. No sense letting a little one die needlessly, and I have never seen it make a difference as far as weaknessess.
I'll wait about 24 hours, and if there is no progress, I get a paper towel, and wet it with warm water, get the egg and a pair of tweezers, and start to VERY CAREFULLY peel away the shell. It will come off in fragment, go VERY slowly, and if you even THINK you see blood, STOP. Put it back in the bator, and wait at least an hour. Repeat. i just make sure to be careful to not go thru any membrane, and keep it damp with warm wet towel. Keep the water out of it's face. I try to get about a half inch of shell off, and let it try a bit before helping more.