Have a state inspector check the hive out. If you have AHB you should (by law) destroy the hive to prevent spread of AHB. The inspector can possibly ID the reason for the die out. For the future you can contact the USDA. The USDA beltsville, MD research center has testing free of charge. Unfortunatley they need fresh samples of dead bees. this website will help you in the future with bee die outs. http://www.ars.usda.gov/Services/Services.htm?modecode=12-75-05-00
The problem could be from the cluster not being able to reach the food stores, pesticides, disease AHB, mold, or even some curuious person opening the hive in winter droping the hive clusters temp to a dangerous level and them not being able to recover. Unless you send in samples every person posting is just taking a wag or swag (scientific wild a$$ guess).
See posting "begginers map to success" for good information and suggestions