OMG that's terrible!
I'm glad that she's O.K now. Maybe it's better she didn't stay the night, because if she did she would've been alone and no one would be able to notice her condition. It's so scary when they go under because anything can go wrong. Is the vet going to reimburse you at all for the spay since it sounds like it was their negligence as to why it got this bas for her?