T.S.
Wow, I am so sorry for your loss.
Depending on what you've told her about HOW babies get into tummies, you should tell her something that fits in that narrative, lets her be sad, but doesn't make her anxious (especially if you plan to try again relatively soon).
My son believes that babies wait in heaven until they are ready to join a family, then they grow in a woman's uterus until their bodies are strong enough to live in the world. So, I would tell my son that the baby wasn't quite ready to join the family and went back to heaven to wait until the right time to come back to us. I would explain that I was sad because I was looking forward to meeting him/her but I will have to wait until he/she comes back.
There is no easy way, but I'm sure you will do what is right for your family, you child and yourself.
Hugs,
T.