Finder question: missing expansion triangle in list view
The window may be using one of the "Arrange By" options. Disclosure triangles are not available in "arranged" views. If that's the case, you can choose View → Arrange By → None or press ^ control⌘ command0.

On Mojave the concept is called "Group" instead of "Arrange". Keyboard shortcut is the same. Menu item is found as follows: View → Use Groups.