When you melt sugar, you turn solid crystals into a liquid. When burnt, that liquid sugar re-crystallizes and sticks to your vessel, creating a mess that's extremely hard to clean.
Filling the pot with water and bringing it to a boil can aid in most cases of burnt sugar. While the liquid boils, stir it to dissolve the sugar and toss everything down the drain.
However, it might take multiple rounds of boiling water to clean stubborn sugar. To expedite the process, cover the layer of sugar with a solution of vinegar and water.
Then, stir in baking soda until the liquid foams, and let the sugar soak in the liquid overnight. The vinegar will make the sugar pliable, and the baking soda will remove stains.
Toss the liquid the next day, scrub the pot gently, and wash it with soap. If the vinegar-and-water solution doesn't work, cover the sugar with a paste of vinegar and baking soda.