It's hard to say, but double check your environmental factors. If she was wet, she may have chilled and died. If she didn't get enough niacin or vitamins she may have died from a vitamin deficiency...please read over the list I (or others) have given you and make sure you are giving your birds a complete diet, with a niacin source and grit. The different feed types are not just snacks, they are vital componenets of a complete diet, and if one or more nutrient ismissing, you will probably have problems. You will not have healthy birds (especially if you keep them indoors, away from fresh grass and fresh bugs)...they need lots of extra nutrition if they are kept indoors.
Also, how many ducks do you have now...? You always want to keep more than one duck; they are a flocking species, and are very social, and need another duck to be friends with. Otherwise their health may suffer, they may stop eating, and may die. Get another duckling of comparable size as soon as you can, and supervise them as they get to know each other so one doesn't pick on the other.