I had almost the exact same thing happen to me back in my college years. Well..... almost exactly. ;)
It was more of a bar than a dentist office and and the anesthesia of choice was whiskey.
But other than that, it was exactly the same ordeal.
I woke up in pitch blackness, stumbled around till I felt a door (out of the bathroom) and found myself locked inside the large college bar I had been celebrating in. First thought was to call the police as quickly as possible before they spotted me first and assumed I was robbing the place.
Thought I would be in trouble, but they called the owner and he came down and was all smiles when he let me out even though it was 4:00 am.
Guess now I know why. Apparently I could have sued him for trauma!
Damn. Too bad the law won’t allow me to sue for something that happened 26 years ago.
;)