I found them smelling yeasty and buttery, sitting a pool of butter but not completely deflated.
You CAN, and I would, but know that the texture may be different that what you'd normally expect. I often thaw frozen croissants overnight for baking in the morning. The time they're out varies from one event to another. I have left them out overlong, too. They're still good, just not as phenomenal as we'd like.