If you want to bring back a task you have previously signed off, from the Mark work page, choose "Signed off" from the Status filter, and find the task you want to restore.

 

Click on the task to open it. On the grey bar on the task, there is a button "Undo sign off". Click this and the task will be restored. It will also reappear on the to-do list of any students who have not completed it.