If you click on the FILTER button at the top of the screen there are a number of options, including the ability to show or hide completed items.
Note:
Completed items will be automatically archived after 30 days.
Archived Tasks 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.
