Damn, that happened to me once as well. I was diving on the east end of Cayman, and took a little trip off the shelf. Something swam by and I got distracted, a little bit later I looked around and - ruh roh, where's the land!
I was gonna pop up to the surface, then I saw a light spot and started toward it, there was some sandy rock area near the shelf.
It was spooky out there. My dive buddy left me and chased a octo. We had 'the chat' once we got back on the boat. I didn't really panic, but looking down was -- unpleasant.
I could see hell.