It happened to Charlotte. We came home from Big Sur to find her sitting on 17 eggs.
Here is an explanation taken from the web:
A hen that is committed to hatching chicks is known as a broody. The state of being broody is controlled by instinct, hormones and lighting conditions. Left to her own devices, a broody will lay a clutch of eggs, then stop egg-laying and sit on them for 21 days (more or less) until they hatch.
Poor girl. I would remove her from the nesting box, then she would just plop on the ground. Then when I am not looking, she finds her way back to the nesting box cause she thinks she has to sit on the eggs to incubate them. Because we don't have a rooster to fertilize these eggs, the eggs will never hatch....but she doesn't know so made it her mission to protect the eggs. I read a couple of blogs to find suggestions on how to break her from this "spell". After the 4th day's attempt, it worked! She is back to being normal again...but tonight I found Maleficent sitting in the nesting box and not moving......another broody hen!