That same exact thing happened to my silkie! One day, after being the healthiest, most active chick we had, his head just sort of flopped over limp. I walked in, and it looked like he had no head because he was sort of leaning on it. We named him Ichabod, after the headless horseman. I really thought the chick had broken his neck or had a brain problem, but randomly he would just lift his head up and be totally fine, eat and drink and such. Then his neck would go all limp again, and he would be really clumsy, walking backwards or in circles. I started holding him in a position that situated his head up and straight, which may not have done anything, but it made me feel a little better. Soon afterwards he got pasty butt, so we had to clean him up often (I think because he wasn't doing much with his head, he had stopped cleaning himself). It lasted about a month or so, but eventually he got over the pasty butt, and started walking around with his head right again. I still don't know to this day what the problem was, and now month or two after he started with the limp neck thing he is completely fine! Also the friendliest chicken we have, because I was holding him all the time checking on him and trying to figure out what was going on.
Keep in mind what was wrong with my chicken (maybe he was just confused, or not too bright) may not be what is wrong with yours. If he really starts to get worse and shows sings of other sicknesses or stops eating and drinking, you may have to look into other explanations. I really hope he is just acting silly and gets back to normal!
