When you confront a villain you have no cards left in your hand but you still control the ones played on the table. Please correct me if I'm wrong so far!

If the villain's attack involves destroying a card you control and you have to destroy one you have already played, my question is, do you lose that power? Do you have to recalculate your power or is it already generated? Obviously, this could mean that you are no longer able to succeed at the confrontation.