From your Library page, or inside a folder, click the switch(es) to Show templates and Workflows that are Archived:

Then, you should be able to see the archived content below.  You may also type the name of the template or workflow in the "Search" box, and when it appears, click its menu icon:

Then, select Unarchive Template:

And to Unarchive a workflow, scroll down on the Library page to the Workflows section, locate the workflow you want to unarchive, click the checkbox on its left side, and then click the Unarchive button at the top: