How to retrieve the deleted test cycles from the Zephyr scale ?

Ouch!  The message received when you try to delete a test cycle is unambiguous - it cannot be undone - and I've not found a way to 'undo' a deleted test cycle.  SmartBear Support might be able to help.  I don't use this feature myself, but another thing you might want to do is to implement Scale security and limit what others can do with regards to Edit/Delete functions.