jwalter, what is your set up for overwintering bees?
this can be salvaged like perry said, just wondering when you last checked the hive?
it's best to remove a dead out as soon as we can, and 'clean' it up as best we can before mold gets too bad. mold is a result of colony death, bees respiration, decomposing bee bodies, deitrus, temps, etc.... and it just keeps on growing; many things in a hive contribute to mold and mold growth after the bees die.
your hive probably died like perry said, because of mites, queen failure, or other reason. the cluster was too small for some reason.