Call your bank as soon as possible, or within 60 days of the error. They may ask you to submit your account information and the alleged error in writing. Generally they have 10 business days to investigate the error, and if they fail to come up with an answer your funds should be reimbursed. If the funds in questions were withdrawn from a point-of-service debit or a foreign electronic transfer, the bank may be allowed more time to investigate the error. In the meantime, however, you should have full access to the funds in question.
Your bank should notify you immediately of their findings. If you were correct about the error, they must immediately finalize the re-credit to your account. If there was no error, they must present in writing the findings of their investigation, and notify you of any funds they have deducted after you had been re-credited.