Folded down so its contents are out of sight. Present and not shown — the state is closed, not empty.
Latin collabi (to fall together, to give way), from com- (together) + labi (to slide, to fall).
Closed and empty look the same from outside and are not the same claim — a collapsed state promises its contents are still there, merely not currently shown, so opening it again should only reveal what was inside rather than rebuild it.