I had this in a young bird a few years back - ruptured air sack. Do the needle method as described above, but not into the crop, its a layer just between the skin and the crop, and get the air out. Try and keep the bird from flying, because it stops the rupture from healing, and it may inflate again - if it does repeat the needle thing.
I had to use the needle twice with the young blue cock I had it with, he was confined to the loft for a week or so and then was back to normal
If anyone has Colin Walkers book to hand, its described much better in there