Father citizenship.

Hello my name is Jaroslav,i live in the united states,i need to know something,im right now trying to help my dad to research if there is anyway to get his czech citizenship,my grandfather migrated to south america escaping when the nazis invaded germany,he has all his documents proving he was a czechoslovakian,unfortunately he died in 1980 in an accident and he was not able to return after the disolution of the country,you know if my dad has the right by blood to get his citizenship?,i heard that once you escape of the country you loose your citizenship so i just want to know as much as possible thanks for your cooperation.