It's amazing to me that we still have paper processes like this. Similar is the situation when you want to rollover a 401k account. Many institutions insist on snail mailing you a check for the full amount, that you must the mail on to the next fund. Ridiculous.
TLDR; call or enter the bank and say, "I lost the cashier's check." They'll tell you what to do and it involves snail paperwork and a few months (possibly) of waiting.