Depends on exactly how much has disappeared.  If the toolbar containing
View has disappeared, press the Alt key before continuing.  If the entire
left hand toolbar has disappeared, click on View, then Layout.  If there is
no checkmark in the box before Folder List, click on this box, then Apply.
Click on OK.

If only some of the folders are listed, click on the triangle to the left of
the
folders group that contains them.