Dan, it sounds like your hive had too small of a population to maintain survivable nest temperatures during the cold part of the winter. It is hard to say why. Many causes are possible, most (in my opinion) likely are queen was laying poorly in the fall resulting in a small population, or, varroa mite infestation which caused the bees to have weakened immune systems, making them susceptible to various viruses, also introduced, or magnified by, varroa mites, which resulted in high mortality rate, and again small population.
Did you treat for varroa mites last fall? If so when, (approximate date) and with what? What was the population like last fall?
It is possible that a swarm could move into the empty hive, I don't know if robbers would discourage that or not. But, you could remove all but some empty comb and a swarm would be just as likely to move in. Then you could add back the honey. The empty comb will act as an attractant. The scouts for a swarm are more interested in the nest size, and height from the ground. The old comb provides a scent to help attract them.
Dennis
