Please help me to undo the bank Reconcilation that was updated in error by the bookkeeper who has left the company

Let me help you undo this reconciliation, @info886.

The feature to undo a reconciliation is available in QuickBooks Online Accountant (QBOA). A ProAdvisor or QuickBooks Online Accountant user can get into your account and revert it for you.

See this article for more information: Undo your client’s reconciliations with QuickBooks Online Accountant. 

If you don't have an Accountant, you can remove an individual transaction from a completed reconciliation. You can do this by going to the Chart of Accounts. 

Here's how to edit the transaction:

1. From the Accounting menu, select Chart of Accounts.
2. Find the account and select Account history.
3. Search the transaction you want to unreconcile.
4. Go to the check column. If the transaction is reconciled, you’ll see an “R.”
5. Select the box and keep clicking it until the box is blank. This removes the transaction from the reconciliation.
6. Click Save once you're done.

Check this article for more information: Undo and remove transactions from reconciliations in QuickBooks Online. 

Here's also a guide to know everything about reconciliation: Learn the reconcile workflow in QuickBooks. 

Stay in touch if you still need help in reconciling your accounts. I'm always glad to help in any way I can. Have a wonderful day!