The good news is that your buck is probably alive and well. The bad news is that you didn't get your buck. I'd chalk it up to experience and go shoot him again. That buck is liable to still be in your hunting area.
I did that same scenario with a large black bear this year. I shot at it at 90 yards broadside with a .50 inline muzzleloader, and the bruin dropped like a rock. Ten seconds later it started flopping like a fish out of water, soon it got it's front legs underneath and started dragging itself off, after a few feet it got on all fours and walked off like nothing was wrong, not much blood, I tracked the drops for 300 yards until I lost it, and that was it. Upon further review I discovered that my gun was hitting 6" high and 2" left, which is another story in itself, note to self, if you drop your rifle on concrete when you are unloading it, shoot it before you go hunting even if it will make you late to your stand. My final analysis is that I grazed the bear across the top of the shoulders and nicked the backbone, just enough to knock it down.