I would make a hole into the air cell so you can see inside. Look for beaks first, if there is one and the chick is breathing, then put it back in the incubator to finish hatching -although if it goes much longer you will probably need to help it. A beak with no breathing means the chick has died during hatch for some reason.
Also look for no beak showing but indications of life such as movement. If there is none then the chick is probably also dead, but if there is then the chick is probably just in the wrong position to hatch. In this case you'll have to slowly open the egg and find the beak while doing your best to avoid active blood vessels in order to save the chick. If a chick is left like this for too long, it will suffocate and die - I just had one during my last hatch that died that way, while the hatch before that saw the birth of Sponge; my hen that I had to open more than half the egg she was in to find the beak due to her being in such an odd position. Chicks hatched this way will probably be very weak at first, and might even still die, but it gives them a chance at life at least.
