Poorly chicken

Please can anyone help?  I have a chicken she's about 5 months old.  Last week she had a cold and i removed her from the rest of the chickens and with a lot of nursing she recovered and even walked towards the pen with the other chickens.  Sunday I put her back in the pen and she seemed a happy although she lost her pecking order and I kept an eye on her.
All was fine until Tuesday morning She tried to leave the coop but was pushed out of the way by another chicken and just lay down.  I picked her up and her neck was floppy and she had trouble holding her head up.
I removed her again and really expected her to pass away but she's stayed with me.  SHe appears very weak and I've put her in a backet and syringe feeding her baby food and live yogurt.  She has moved today but still having problems holding her head up.
Please any suggestions