Nearly all hits to an internal will kill an animal. Just about all animals that are gut shot die within 24 hours. As many experienced hunters like some on this site will tell you, most animals will bed down within a couple hundred yards of being shot. Assuming the deer isn't spooked he will probably lay there and die of internal bleeding. If you follow the path that you last saw him take and can't find him, I would suggest doing a criss/cross pattern with a radius of a few hundred yards. I would bet that he is in that area, you just have to find him. Good luck, let us know how it turns out.