I am (was?) a single hive beekeeper (In Arlington, VA).....and the
hive died...
 
Today was the warmest day for two months, so I went to check my bees.
They are dead, the hive (two brood boxes)  is empty....  Dozens, but
not hundreds of bee carcasses... which means some left and died
outside the hive..
 
There is a lot of honey still in the hive.... so they didn't starve.
I took no special winter precautions....(the bees survived last winter
ok).. I see no evidence of disease,  although I am not sure what I am
looking for.. There is a little mold on the interior of the
 brood boxes, and on the ground under the hive...
 
Should I assume that the cold snap in January did them in, or are
there other factors?
 
Can I simply re-introduce a package of  bees into the hive in
April?  If so, how do I keep ants and other critters out of the hive
between now and then?
 
How can I be sure I don't have some disease that would infect the new
stock??