Poorly chicken

I have a hen who is about 4 years old, she developed a wonky neck last year, which only became obvious when she ran, but the last two days it has become much worse and she dosn't move much and she keeps closing her eyes and her neck goes right back, I thought I was only to loose her this afternoon, but she is still hanging on, she is not distressed but I just want advice as to what to do for the best please.

I agree with what andy says or if you think she is suffering could you dispatch her yourself?