So the air nozzle isn't moving back to allow a bb into the chamber?
Look into the hole and dry fire the gun a bit. If the nozzle doesn't move at all, it's probably fallen off the tappet plate, meaning either the air nozzle, tappet plate or cylinder head is broken. If the nozzle just moves a tiny bit, the problem's with the tappet plate. Either way you have to open up the gearbox and replace the broken part.
This happened to me on my JG HK416 a week or so ago so it's quite fresh in my mind atm =D