Tell her you love her. Tell her that there is a hope for her to see you again. Tell her that no matter what, nothing was her fault. Kids blame themselves for bad things because they don't understand how things work- so the only explanation in their mind is that it is *their* fault.
Whether you are religious or not, talking about death with her NOW is important. This can help:
If you don't go to that link, then adapt these practical points from it:
IMPORTANT POINTS TO CONVEY
If a loved one has died, you can help your child overcome inordinate fears about death by making sure he understands the following:
She is not going to die. (this is geared toward very young ones who may fear death is contagious)
If something happens to you, she will not be abandoned; relatives will look after her.
Once dead, a person is no longer suffering.
The deceased person will not be forgotten. You might say, “You cannot see Grandma anymore, but you can keep memories of her in your heart.”
Remember, saying nothing to your child will not reduce his anxiety. On the contrary, it may only serve to give his imagination free rein.
Please put geekdom aside. She will always remember how awesome of a geek you were, but she'll especially appreciate that you were an amazing father *first* and that you loved her.