I had a similar situation with one chick in my first hatch. It zipped and had a leg out for over a day. The membrane dried out and it was obvious he was getting more and more tired, and eventually his little leg started to shake uncontrollably. I figured I could not make a horrible situation too much worse, so I took the little guy out and tried to gently moisten the membrane with a damp scott towel. The top of the shell was stuck to his back; the membrane was very dry as were his feathers.
I ended up disenfecting a pair of tweezers and SLOWLY and in TINY pieces break off sections of the shell. One piece did not want to come off, so I just left it on.
This little guy lived but got off to a slower start than the other guys. It seemed to take for ever for his feathers to look "right", but he is alive and well and now you would never know the rough beginning.