You can view completed tasks and re-open completed tasks at any time.
From the Task Flow view
1. At the bottom of the Task List, look for the tasks completed section.
2. Click on tasks completed to show a more detailed view of what was completed.
3. Click on Reopen to bring the task back to the list. You might have to refresh the page for this change to take effect.
From the Task List view
1. Click on the Task List name or on the tasks completed link below the Add task button.
3. Click Reopen beside the task if it is not yet completed.