D.K.
I went through this with my 5 year old when my 21 year old nephew died in a car accident. What has worked for me is to tell the truth. The truth is that you will die someday, but it's really unlikely that it would be anytime soon. My son would cry about this prospect and I would hold him and comfort him. This went on, usually at bedtime, for a couple of weeks. By telling her the truth you are sending the message that you know she can handle it (someday she will have to handle it)and you are there to hold her while she is sad. Besides, kids know when we aren't straight with them. Just be straight and let it be okay that she is sad about it.