Begin by clicking on the module the discussion is in, then click on the discussion itself. It will display on the right of the page. Click on the switch at the top of the page that's labeled "Hidden" 

After you click on the switch, the hidden eye icon will disappear and the switch will now be labeled "Visible"

To make discussions visible from the Forum level click on the "Tools" dropdown, then "Discussions." Look for the "Hidden" icon beneath a forum - that means that all topics in that forum are hidden. To make them visible, click on the caret next to the forum, then click on "Make Visible to Users."