If you have other hens of the same age that are laying well, the issue is probably not dietary. You may just have a hen who decided to quit laying eggs. I've had a couple of hens that were great egg layers, but who just stopped laying when they were 15 to 24 months old. Some became internal layers (RIR), but I'm not sure about the others as they seemed healthy, but just stopped laying (golden laced wyandotte, Easter egger). After months of no egg production, I finally decided to cull them.
With many hens, this type of decline happens when they reach 3 or 4 years of age, but with others, a dramatic decline may happen much sooner. It may be genetic, or it may be some type of internal disease or disorder. Just curious - what breed is she?