Healthy appearing 1YO hen died

[DMoody]

Yesterday evening I last visited the girls and as usual they were all their normal curious selves, following me, eating, watching every thing I do, etc. nothing unusual from any of them. Very active. This morning one of the speckled Sussex was lying on the coop bottom right under one level of roost. They have all begun a small amount of molt but nothing major. I don't understand it. Any ideas.
Also no one tends them except me, the rest all look good this morning. I watched them all devour fresh corn and grains.

Thanks,