When you complete the task in My tasks, it disappears from this list. But you might need to find it later to view or reopen task. So, to access already completed tasks, you need to choose "Completed" filter instead of "Active" on My tasks list.
- view completed tasks list. You can find completed tasks by changing "Active" filter view to "Completed".
If you'd like to reopen task, you can do it in Completed tasks list - just press the checkbox on the left of task name. The task goes back to My tasks (Active) list.
It is also possible to reopen task from Task details page. Click on the task name to access it and you'll see button "Reopen" in the right part of the screen
Next article - How to change task due date