I am new, so hopefully some experienced people will chime in, BUT....
That happened to one of my chicks last night(!)... Hatched on her own although took quite a bit longer than the other eggs to pip, then about 16 or 17 hours from pip to zip, then about 1 1/2 hours from zip to hatch; all in all a significantly slower progression than all of the others (except for 1 remaining egg which I suspect is dead).
What I did: Pulled her from 'bator (was only risking the "suspected dead" egg, all others had hatched out), was SUPER QUICK about it anyway, "just in case" last egg viable. Temp dropped by about 1 degree, humidity by about 3% but both came right back up within 4 minutes.
Held her on my lap on a paper towel & sterile guaze pad, applied VERY GENTLE pressure to bloody area, ran a blow dryer on LOW to dry her off & keep her warm (be very careful, was still hot enough to burn her delicate skin... keep it moving & keep your hand in airflow to monitor temp!). She lost about a "quarter-sized" (on paper towel) drop with a small clot, and a couple "dime sized" drops of blood as well. Seemed like a fair amount to me for something so small (!!).
Stopped bleeding within 10 minutes or so. Weak but alert. Stronger this a.m.... Looks like she will be okay (I HOPE!!!!!).
Good luck & I hope someone else with experience chimes in !!!!!!!!!!