An admin or leader has the option to reopen a task that was already signed off.
You can do that by clicking on a round arrow in the status part of Task details. When reopening a task, you will need to provide a comment and it is by default marked as a problem and can be unselected.
The assignee will receive a beat notification, but it is advisable to mention them in the comment so they also receive an email notification.
This can only be done from the Desktop version.