You can collapse one node with code like this:

ThisForm.TreeView1.Nodes("SampleNodeKey").Expanded = .F.

This will collapse only that node. If you want to collapse all nodes, you can collapse root node. In this case all other nodes will be not visible, but internally will be not collapsed. I.e. if you expand root node, they also will be expanded. For every node, which you want to be collapsed, you must write code like above.