If a building or other structure collapses, it falls down very suddenly.

If something, for example a system or institution, collapses, it fails or comes to an end completely and suddenly.

If you collapse, you suddenly faint or fall down because you are very ill or weak.

If you collapse onto something, you sit or lie down suddenly because you are very tired.

If something with air inside collapses, it falls inwards and becomes smaller or flatter.