How to answer my granddaughters questions about her parents?

My granddaughter (gd) is being raised by my ex and his wife, the parents (my son and his ex gf) were not in a position to raise her. Long story short my gd has a relationship with her dad but not with the mother who has had 2 more kids and has not had any contact with my gd in about 3yrs. Now my gd is 8 and has asked her dad if he knows where her mom is? We new the questions were going to come up eventually but I am not sure how to answer her if she poses the same question to me. My heart is torn for her and because of her age this needs to be handled with care. Any suggestions are welcome!

Answer honestly and age appropriately, I grew up in a similar way and it was always easier to hear the truth. My mom was a drug addict at the time and my grandmother just explained that mommy was sick and until she got better granny was all I had.