This might sound harsh, however, as your slave she is your property.
It's her job to take excellent care of your property - herself included.

If she can't find enough value in herself to play safely, then you must put that value on her.

She is a precious gift, a woman to experience hot nights of passion, love and warmth and she sure as heck can't do that if she's hurt herself.

Perhaps a not so gentle reminder that for her to remain with you is that she must take care of her mind, body and spirit and leave you to take care of her sensual needs. (Or she can do self bondage under certain circumanstances, with your permission, safely, etc.)

She might need some hard rules and punishments to understand that you are serious and that this type of behavior cannot continue.

Either way, a good deep conversation is needed. Communication is key.
You might explore what she was seeking that drove her to such a dangerous self experiment and be honest about how it made you feel to find her in such a predicament. She might have thought she could take that big of a toy
or wanted to be rescued. Deep down, she could have been testing you.

Some suggested rules of communication for these and other dicey situations:
1. Ask a question and listen to the answer.
2. Repeat back the answer to make sure you understand what she said.
3. Use I statements...when you do this, I feel like or I think "whatever".
4. Forgive
5. Ask yourselves what did we learn from this and move on.

Good luck and best wishes.