Are you absolutely positive she hasn't laid any eggs? Can you tell the eggs apart? There are many reasons a hen might not lay, stuff like a damaged ovary, disease, and not enough protein to name a few.
If you are feeding a layer that is 16% protein switching to a higher protein ration can sometimes jump start the system as it takes a lot of protein to make new feathers and to make eggs. When I switched to an All Flock ration a few years ago my production picked up and a 8 year old hen suddenly started laying sporadically again, so she could be deficient.