Tasks that are deleted or trashed are not gone forever. Instead they are placed in the "trash" for 30 days, giving you a chance to bring them back should you decide to reactivate them later.
Should you wish to bring any item back, you simply need to click the "TRASH" icon in the menu. Click the bulk edit icon to enter the bulk edit mode. Then select the items you wish to put back. Finally, click the "Put Back" button to put back any items that you had selected to keep.