We don't want you to have this experience with your bank transactions in QuickBooks Online(QBO), @kass777. I'll help you sort this out.

Firstly, when your bank account is disconnected from QuickBooks, the only transactions that will be removed are those in the For Review section.

For your existing open balance of $20,000, reconnect your bank and Categorize payments toward your open invoices to fix the balance.

Please remember that QuickBooks only syncs the last 90 days of transactions. If you have older transactions, you'll need to import them manually.

Regarding the mileage tracker issue, we can do basic troubleshooting such as clearing your browser's cache and cookies as well as any temporary files. This might fix any malfunctions. If issues persist, consider using other supported browsers to ensure QBO runs smoothly.

Next, for any mileage that wasn't tracked, you can enter it manually to adjust the deductions accordingly.

You can also check your audit log to see who has made changes to their QBO company, especially if their bank account shows as disconnected.

Once everything is okay, you can start with Reconciling your account to match your bank and credit card statements in QBO.

If you still have more questions about your bank transactions, always know that you can come back to this thread. We're here to help.