From a paperback collection of short stories, picked up from an airport store while waiting for a flight in the mid 1980's.
A couple are on a long trip (maybe multi generational) with a ship full of hibernating passengers at sub-light, when attacked by a unknown ship, the attackers energy weapon cannot get through their shield, the attacker then use a faster than light weapon that can, after crippling them the aliens want the shield technology, after using the shield to get rid of the attacker, and knowing that faster than light travel is possible, they turn around and head home. No need to waste more time with sub light travel.
Thanks for any help.