When you start a new Sage 50 file you will have to populate all of the General Ledger accounts as at the date of starting. This is normally done by entering the opening balances while in History mode. To get these opening balances you would use a Trial Balance from the old set of financial records or you would use the trial balance provided by the accountant who produced the year end reports and filed the taxes. 

In your case, you cannot just use the opening balance on the bank statement as you do not know what deposits and/or cheques are outstanding from a previous period. When the prior records are not easily obtainable I would wait for a couple of months to verify that all entries in the bank are either current and in your records or they are from the prior period. Once this is determined then you can add the prior entries manually, only for reconciliation purposes, when setting up the first account reconciliation. This will then allow you to determine the opening book balance for the bank which is probably different than the amount on the bank statement.

This is not an easy task until you have sufficient information. Good Luck and happy hunting for all the necessary transactions.