So sorry as well.
Just be honest, simple, and clear. Like gymtigermom said, if you can relate it to a pet or friend's pet (or relative) dying, that will help. 
Answer any questions she has about it simply and honestly. Regarding afterlife questions, I would suggest to only talk about Heaven, etc., if you are a believer yourself. If you're agnostic, just be honest and tell her no one knows for sure what happens, but [insert your beliefs or hopes]. 
My kids, while they've been very young, have lost their grandpa, a young cousin, and pets (and friends's pets). They know that people and animals become old, become ill, or become injured, and bodies won't work anymore. All people and animals all die someday. We have always stated this as a fact of life, and let grief proceed as it may.
Wishing you the best. s