well it sounds like you already did the right thing by backing out. Go back and look for him in the daylight, usually a gut shot deer will find a thick place nearby to hole up. If the shot hit something fatal hopefully he died peacefully in his bed over the night. Just got nice and slow when you are tracking him, if you don't find a lot of blood (or for that matter, any blood) you will have to make a grid and just search for him by sight.