There could be a few reasons why the balance is showing as zero:
You have not reconciled the previous month.
You don't have any transactions in that GL account for that time. Go to Chart of Accounts and select the GL you are trying to do the bank reconciliation for. Search for the dates needed and confirm that you have transactions.
You have not set up your bank reconciliation groups. To see if this is set up go to tasks>Bank Reconciliation>Manage Groups and make sure there is an owner and a property assigned.