Hi, normally we would try to remove any foreign object that fell into the crankcase rather than take the chance that it will fall harmlessly into the oil pan. You did not mention the year and model of your vehicle but I suggest that you remove the valve cover if possible to try to remove the ring. Chances are no harm will come if you leave it there. The ring is so small that it probably could not interfere with any of the moving parts.
What else do you need to know?