Hopefully, you have kept some statements, a pass book, or someother piece of evidence that you did indeed have these funds. Unless you can prove that, you are hooped.
Next, I would make an appointment with the manager of the bank. I would lay my evidence before him that I had entrusted my funds to his bank; and, it is up to him to be responsible for what happens to them. Inform him that if he does not take some action to find this missing money, you will taking action to report your missing money to the police and to the General Manager of the bank.
When you arrive home, make notes of what went on and send the manager a synopsis of the meeting by either e-mail or snail mail. End the letter by stating that if his recollections of the meeting are different than what you have outlined, he has until XX date to respond.
If you are not satisfied with his responses or help, go to the police and the head office of the bank involved.