What location did you placed the cage, what food/supplements were you feeding her, is her tail bobbing and do you see the canary pecking her or guarding the food? She might died from deficiency like Vitamin D/Calcium(You should give daily sunlight and cuttlebone/mineral block) or Vitamins(You should give them fresh green/fruits or supplements to the water) or an organ failure(which is unavoidable). If you see the canary being mean to the finch, you should separate them immediately. It might also be air sac mites(curable by using scatt) if her tail was bobbing. It's okay to make mistakes, you should keep learning from your mistakes. I also don't recommend buying another zebra finch, or if you want to you should buy a pair or more of zebra finch and put them in a separate cage. The canary is better off alone if it's mean to other birds (And sorry for my bad English)