If there are no small scratches or other marks and it's just matte, you can first try cleaning with diluted ammonia (mix 1:10-1:20 with distilled water to avoid lime spots), wipe it off while it's damp, and then polish with a polishing cloth.
If it doesn't improve or scratches are visible, you can use wet sandpaper for stone and an orbital sander, but depending on the grit and quality of the paper, it might take some time to achieve a shine on the marble.