You may never get it back, but wet sandpaper used often on car finishes in finer and finer grits will help. You might want to go up to 2000 grit, depends on how shiney the original stuff was. I'd stay away from power tools - first, ones that can be used wet are expensive (compared to homeowners versions), and it would be easy to go too far.
Check out www.johnbridge.com. They have a restoration section on tile (even if this is slab, it would be the same for a marble tile).