You probably opened another view by accident. There are the four "2D 3D Script AssetLib" at the top, as well as tabs directly bellow them, where you can switch from one scene to another. Additionally, the Node tree shares it's location with the Import menu, with their tabs on the top just below the menu bar.
Another possibility is that you closed your scene, by clicking on the X of it's tab.
If none of those where your issue, try taking a screenshot and sending it.