Let's update your bank's sign-in info to re-establish your bank connection. Let me walk you through the step-by-step process.
Go to Banking, then Banking at the top.
Select the account in question, then Update. QuickBooks Online tries to connect to your bank to manually update your account.
Type in your updated username and password. If an error will appear, click Reconnect my account. Tick the Pencil (Edit) icon next to the bank name, then Edit sign-in info.