Group wikis can be undeleted by any of the group members or the instructor, class wikis can only be undeleted by the instructor. To undelete a group wiki, on the main group wiki page, click the "View Deleted Wikis" link.
This will open a pop-up window that lists all of the previously deleted group wikis. Click on the title of the one you'd like to undelete.
This will refresh the pop-up window with the content from the deleted group wiki's main page. If you want to undelete it, click the "UnDelete this Wiki" button. If not, click "Close this Window." Selecting "UnDelete this Wiki" will close the pop-up window and return you to the group wikis main page, where the wiki will now show up in the list of available group wikis.
To undelete a class wiki (remember, you only have access to this option if you are the instructor), on the main class wiki page, click the "View Deleted Wikis" link. From there, follow the same steps as above.