I followed a tutorial on how to hide just the outliner but the tutorial never showed how to bring it back. I can only quit Blender and restart to get it back.
- Click and drag the hashed corner overlapping the area you want to shrink in order to make way for the new area.
- Click the drop down on the far left of the Header of the newly created area
(optional) RMB anywhere on the header and select Header > Flip to Top.
You can read more about the interface system in the manual.
This depends what you did to hide it. Have you dragged another window over it, and removed it from the layout by that? Or have you just changed the editor type ?
Have a look at the little handler up right and downleft of every window. Pull it out. Then change the editor type for the new created window to the Outliner. That's something that you can do in the little menu at the very left in the header.