If you need more than 90 days of transactions, you can export additional historical transactions from the bank and upload them into your QuickBooks company.
To do that:
Click Banking from the left navigation panel.
Pick File Upload from the Update drop-down menu or Upload transactions manually if there isn’t any bank connected to QBO.
Select Browse and choose the file you downloaded.
Choose an account from the list, then Next.
Follow the on-screen instructions to map the fields.
However, if the bank entries were there already and suddenly went missing, I recommend updating your account manually to refresh the connection between QBO and your bank.
Here's how:
From the left toolbar, select the Banking menu and tap the Banking tab.
Click to highlight the account you're updating, then hit the Update button.
Also, here are some helpful articles about fixing online banking issues that you can keep for your future reference: