You can set your Prioritizer to show outstanding action items and completed ones, to show only the items you've tagged as "Musts," or to hide anything you've marked done or snoozed.

If Only "Musts" or Hide Done + Snoozed is toggled and you prematurely mark something done, it will disappear. By toggling the Everything view, you'll be able to see what you've marked done. You can click the check mark to un-complete the task and make sure it returns to your Prioritizer as an unfinished item.