L.C.
Oh gosh, M., I am so, so sorry. She wouldn't have a great concept of death, it won't make a lot of sense to her. I would go to the children's section in your library and look up children's books about death and dying. Find the one that you think says it the way that you think is best and check it out. Read it to her and tell her that's what happened with Daddy or that's where Daddy is, but that he still loves her very, very much.
My heart breaks for her and for you, having to help her.
God bless you and I'll be praying,
L.