Boil chicken breasts and celery in 4 cups of water for about 30 minutes. Remove from heat and remove chicken from water to cool. When chicken is cooled, shred and set aside.
In a large skillet, melt butter or margarine. Drain mushrooms, reserving liquid, and add to skillet. Heat thoroughly, seasoning with salt and pepper. Add evaporated milk, reserved mushroom liquid and sherry, stirring well. Add cheese a few slices at a time until thoroughly melted and blended. Add shredded chicken and bring to a slow boil.
Preheat oven to 350 degrees (175 C). Grease a 9x13x2 glass baking dish. Put in noodles at about a 1 1/2 inch depth and pour chicken mixture over noodles. Stir to coat. Top with cornflake crumbs and paprika, cover dish and bake for 25 minutes. Uncover dish and bake for 5 more minutes. Serve warm.