High spine.
ALWAYS put another arrow in them in this situation ASAP.
The reason why is obvious - learn from it - we all do.

Sounds like you nipped the spine, I doubt hes dead at all. Apparently your carefull aim wasnt carefull enough. Youre more than likely not going to find any blood with a shot up as high as it was. And you should have arrowed him a second time. Nothing that hasnt already been said.

just happened to a buddy of mine on a PA 6 pointer over the weekend...all indications were he hit just below the spine, but enough of a blade to nick the spine...the deer dropped, legs presumably out of commission...my buddy waited 20 minutes as he climbed his self climbing stand down, then the buck jumped up and ran off...another arrow would have done the trick in his case and your's maybe too