All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
If the bank where you save your money deals with interest and usury then it is an obligation on you to close your account in it and withdraw all your money from it because it is not permissible to open an account and save money in banks that deal with usury and interest. If you save your money in these banks, you are helping them in sin and transgression, unless you are in need for doing so. When the necessity is over, one has to withdraw all his money from such banks.
As regards the money that are saved in this bank and you cannot withdraw it, then there is no Zakat on it if it does not reach the minimum amount liable for Zakat. However, if you have other money whether it is cash money, gold or silver or other tradable items, and by adding them to that money they reach the minimum amount liable for Zakat, then you have to pay Zakat on it on one lump sum when you will be able to obtain the money that you saved in the bank, because the Zakat that you owed still must be paid.
Allaah knows best.