A candle should not have damaged a porcelain enameled steel or cast iron sink! A burn mark should come off easily with some soft scrub and a sponge.
But if the candlestick was hard and/or heavy, it may have chipped it. In that case, clean out the bad spot and paint on some sink repair paint. It comes in a very small bottle at a hardware store. It will probably take a couple coats. If you do it carefully, it will give a good looking and permanent repair. You will also need some very fine sandpaper.