I was wondering if you tried to help already? Usually chicks pip and zip, meaning they crack the shell in a circle on one end without really completely removing any pieces until they are actually ready to push all the way out. This chick is missing a huge chunk of shell by its face so it cannot move around the shell anymore (pushing against the shell with their beaks helps them slowly manuever around and crack as they go). You are going to have to help this baby. I would not put it back under your broody and leave it or it won't make it. If you have a bator fire it up, if not you will need to use the broody for periods of time. You need to slowly help it hatch. It is important you keep the baby warm and moist, use a spray bottle with warm water or a wet washcloth. While you remove shell look for any blood. If there is blood you will need to stop and wait for several hours, it means the baby is not ready to come out yet. If you keep going you run the risk of it bleeding to death or being hatched prior to absorbing the rest of the yolk. If you don't have a bator than make sure it is damp and put it back under broody to keep it warm. Keep an eye on her to make sure she is keeping the baby warm and doesn't peck at the baby, especially if there is blood. Than repeat again. When the baby fully hatches there should be no blood running through the membrane around the egg. Good luck!
