I am soo sorry for your untimely loss. I am sure that your vet is devastated too. Having worked for a vet for a while when I was younger, I know that it is very rare for this kind of thing to happen, but it does on occasion. I saw one vet, a middle-aged man, work frantically to revive a little dog like yours and when he couldn't he broke down and cried. That dog had been in for a routine dental appt. and just had a horrible, tragic reaction to the anesthetic.
Sending huge hugs your way, I am sure you are in total shock. I feel so bad for you. :no: