I'm sorry to hear it Marla. I wouldn't blame yourself for this. I know it's hard, but what happened wasn't out of negligence on your part. We know you all loved your critters, and wouldn't let harm come to them if you could possibly help it.
What may have happened is Jez may have cracked open the Sterilite and pushed it aside. Phoenix did that with my slide-top cage, and that's how Roo got out a few days ago.
You tried your best to give her a good life, and a peaceful end. Sometimes, that's all we can do. My thoughts are with you.