I concur with the surgery and anesthesia comments. I have had more than thirty major operations (ones that require more than local anesthesia). Without going back and looking at my records, I believe that more than twenty of those required general anesthesia.
The longest one required about twelve hours. This is a very long time to "be under", I was told. What I remember of it is the anesthesiologist asking if I was OK. I replied something to the effect of being quite used to it by now, but still a little scared because of the length of time it would take. He then told me that he had all kinds of good drugs to take care of that and I wouldn't be scared of anything in just a few seconds.
Then, as if by magic, I was lying in a different room with people sitting and standing around my bed. I was quite groggy and very uncomfortable with the breathing tube down my throat and tied down such that one arm is all that I could move.
There was no sensation or memory of anything between. It was exactly the same with the other times I was put to sleep. I suppose this is what it is like to "not exist".