Based on your description, it sounds like the creatives you were referring to probably became inactive–we provide some context about the creative lifecycle that discusses this
here. After a long time passes where a creative is inactive, it eventually is deleted. Assuming that has not happened yet for these creatives, you should be able to reactivate them if a bid is placed with them. While in the inactive state, creatives have limited visibility in the API–e.g. attempting to create a new creative with a creative ID that is identical to an inactive creative would fail.