Check your thermometer to make sure the temperature is correct. If it is reading right then the embryo was weak or defective. It happens. I had a single call duck egg in my incubator which died yesterday. I know my incubator did not malfunction. It happens just as some human pregnancies end in miscarriage; sad but it happens. Try again and if this is something new to you, reread about how to use your incubator and look at egg development charts, found online, so you know what to expect. Sorry and good luck with your next eggs.
What kind of incubator do you have? Does it keep a stable, unchanging temperature? Mine keeps a temperature to within 2/10ths of a degree; 1/10th of a degree above and below the desired temp. Does yours have an automatic turner or do you turn manually? Mine has a turner, but I prefer to turn the eggs by hand three times a day.