Did you add any yeast nutrient? Sugar does not provide all of the nutrients required for yeast growth and health. If you did not add any, then your yeast probably got worn out and quit working. You can buy yeast nutrient products at beer, wine, and distilling supply stores. You can also just boil some yeast in water to kill it, and add it as a nutrient. If you buy nutrient products, the directions should tell you how much to use. For your current batch, you could add nutrient, and additional yeast, to get the fermentation restarted. A pure sugar wash should finish fermentation well below 1.000.
25 lb of sugar should produce 13.46 lb ethanol when fully fermented. Water has a density of 8.33 lb/gal @ 68°F, so 15 gal weighs 124.95 lb. Your final wash should be 100% * 13.46 / (13.46 + 124.95) = 9.7 wt% sugar, or 12.1% ABV. The SG would be about 0.984.
Brew on