Tasks that are deleted or trashed are not gone forever. Instead they are placed in the "trash" for 30 days, allowing 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 tap on the "trash" icon in the lower left menu. Tap the bulk edit icon to enter it's bulk edit mode. Then select the items you wish to put back. Finally, tap the "Put Back" button to put back any items that you had selected to keep.