Tasks are stored within the archive tables for historical purposes.
If a task has been archived accidentally:
Click on the task in the Archive Table
Change the status to 'In Progress' or 'Not Started'
The task will then move back to the main table.
Every year or so, you might want to clear a few tasks from the Archive Tables.
To delete tasks from the archive table:
Go to the Archive table.
Select all tasks by clicking on the square next to the column heading, or select a few tasks by clicking on the square next to the row.
Go to Actions
Click Delete Selected.