What happens if a cashier’s check is lost or destroyed before being deposited?

Its just a guaranteed cheque. If it is lost, you simply go to the issuing bank and ask for the lost one to be cancelled and a new one to be assigned. There is a fees associated for canceling by some banks. You will sign a document that would say, if the 'lost' one is accidentally encashed, etc. the bank will not honor it, and if it has to honor it - it will deduct the amount from your account without you objecting to it.