li-ion battery?

i own a hitachi dv18dl drill and its only been used a few times since ibought it less than a year ago no major work. I keep it in the case in a cubord , and in a dry garage, it was working fine when i put it away ,but now the charger says the batteries (both of them ) are dead ,is it a case of cold in which case there must be lots of people who have suffered the same problem, or do i have a case for replacements on warranty do you think ?.