You can reopen a task that you already marked as completed.
To reopen a completed task, filter your Work view to see "All Completed Tasks" and select the task you wish to reopen.
You can also open the Project Details and select a completed task from the Project Task list.
The Task Edit Panel will open and you can select "Reopen Task".
The task will go back to being "Active" and you can pick up where you left off.