On the Dashboard, please click on the Cases button in the group of colors buttons.  This will generate an excel file.  One of the column in the file indicates whether the case is archived or open.

Alternatively, click on the Cases (Projects) tab, then enter % in the search box.  All cases will be displayed in the left side bar below the search box.  Archived cases are shown in different color.

If you remember a few letters of the case name, entering in the search box will bring up the case.  On the right side, click on the Edit Case tab and use the checkbox there to re-open the case, is so desired.