Thanks for coming to the Community for help. I'll provide some information about disconnecting a bank account in QuickBooks Online (QBO) and how it works.
Disconnecting a bank or credit account from QBO will remove the unaccepted or uncategorized transactions from the bank feed. However, those that were added, categorized, or reconciled will stay in your account. This also simply stops QuickBooks from downloading new transactions. You can click this article to see additional details: Disconnect or delete accounts connected to online banking.
For now, there's no option to directly undo the disconnected account in QuickBooks Online. Meanwhile, you can reconnect the account to start downloading transactions again. Then, categorise them to avoid duplicate entries.
Please refer to this article to view details on how to reconcile your accounts so they always match your bank and credit card statements to ensure your account stays accurate: Reconcile an account in QuickBooks Online.
Please post again in the Community if you have any other questions about bank feeds, I’ll be more than happy to help. Take care and have a great day!