Savings was withdrawn by the unknown

Maybe someone could help us and give advice. My fiancé went to the bank to withdraw money from his passbook. Unfortunately, the bank informed him that his bank account has 0 balance. It was all confusing because he rarely withdraw on that account and it is his savings for 2 years. What is the best thing to do? He is really frustrated and depressed because his savings were withdrawn without bank notice and the bank said to him, the withdrawal doesn’t indicate any location and thinks it was hacked. It is a reputable bank and it is disappointing that they lack security. 😔😔😔😔😔