"The autumn was cold."
Translation:Осень была холодная.
Cases don't depend on a tense. Осень была холодная, осень - [есть] холодная, осень будет холодная, all adjectives (холодная) are in nominative case as the defined subject (осень).
You mentioned "холодно" which is not in the answer here, so I suppose your answer is different. Холодно is an adverb derived from холодный, they are not declined by cases. Not sure what answer you had.