All of the GPs passed away before our DD was a few weeks old, and she has always struggled with understanding it. However, thanks to a fabulous explanation I saw on MN, I think she has now started to get to grips with it; she is also 5yo, so maybe it will help you too.
We explained that being alive is a brilliant thing because you get to do all kinds of great things like ride your bike, draw, have hugs, swim, etc. And that all living things, people, animals, etc get a turn at being alive. Some people have a long turn and some people have a short turn, but the important thing is to make the most of it when it's your turn. She seemed to really get this explanation, as at this age taking turns is pretty well embedded.
She also asked about what happens to your body, and we told her that when your turn finishes the little spark inside you that makes you, you (that some people call your soul) leaves your body, and then you don't need your body any more, so you leave it behind, just like taking your clothes off at the end of the day. She asked us if your soul goes to heaven, and we said some people believe that, but nobody really knows, because the only people who know for sure are the people that have died, but that she could decide what she believes as she gets older. She seems quite happy with that explanation, but you will know better whether you think your DD will respond well to that or not.
One last thing, don't be surprised if she asks you the same thing repeatedly over the coming weeks. We found it very upsetting at times, but she was really just trying to make sense of it all. Good luck, and my condolences to you 