One of the bank accounts have lost a few months data. Please advise how to retrieve the lost data.

Thanks for joining us here in the Community, @lucyphang.

To retrieve the lost transactions on your bank account, you'll need to upload them manually in QuickBooks through WebConnect. It allows you to download the bank entries from your financial institutions' website and import them into QBO afterward.

To give you the complete details on how to perform this process, please see this link: Manually upload transactions into QuickBooks Online.

You can also check out these articles I've added below for more information on what to do if transactions are not downloaded automatically and if you encounter any banking errors:

• What to do if you can’t find downloaded bank transactions
• Fix bank upload errors in QuickBooks Online

I want to make sure this is taken care of for you. If you have additional questions or concerns while working in QBO, leave a comment below. I’m here ready to help you.