What happened? Not being able to see the air cell has two possible explanations, as far as I can see. The first is that the chick has broken through the internal membrane and is now filling the air cell. The other possibility is that the egg has some kind of defect or has been damaged in some way, but if you candles earlier in incubation and saw an air cell this is not likely. So I would say the chick is in there trying to get out!
If you havent already done it, this is how i would make the hole. Candle again to try to locate a less dark spot on the fat end of the egg. Then try using a sharp pointed tool (like one side of a thin pair of scissors) to scratch lightly in one spot, twisting the point into the scratch a bit until it looks chalky and then the outer membrnae is exposed. Then pinch that out with tweezers to make a hole.