Archived items can be found by using search in the Archive view from where they can be viewed and unarchived if necessary. Click the More menu option in the View Switcher to find Archive View, then either search or scroll through the items shown.
The Archive View will show the results in table format similar to that shown below:
Clicking the "View" button on a row will allow you to view the archived item. Note that when it's in an archived state, you can only delete the item or unarchive it (at which point anything can be changed).
Sortd will only let you unarchive an item if you have not exceeded your board or list limits as per your subscription plan.
In the case that the list that the item was on has been deleted, you will need to choose another list to unarchive it to by clicking on the list (shown as "Project Board - Done" in the image above) and selecting an alternative list. This option is only available if the original list has been deleted, and the list name will subsequently be shown as "Unknown" in black.
Once an item has been unarchived normal functionality will continue.
Note that unlike the normal Table View or normal Board search the page numbering is not accurate and you may have to keep scrolling to find an archived item, so it is better to use the search bar when in Archive View if you know the name/title of the item that has been archived. The reason for this is that the Archive View loads data incrementally from the server and hence does not actually know how many items have been archived.
Similarly, the ability to sort and filter the search results is limited to the data that has been loaded in the browser which may not be all the archived items, so once again searching for the item title provides the fastest way to find an archived item.