Have you lost your folder pane that lists all your folders down the left side? If you do not have a menu bar press the alt key to make it appear. From the menu bar select View-Layout-Folder Pane Now click on the Inbox.

No list of folders appeared.

So you are saying that the Folder Pane is checked but still not showing?

If so move your mouse to the left edge of the screen. Watch for it to change to a double headed arrow. When it does, left click, hold and drag to the right to uncover the folder pane.