Why is that the bank connected to online banking went missing? What would be the possible way to get them back?

Try opening your QBO account on any private/incognito browser. Can you find it?

Hi jam030108,

Thanks for reaching out here.  Welcome!  It's important that you have an complete view of your linked financial institutions in QuickBooks Online.  I'd be happy to provide more information so you're back to business as soon as possible!

In addition to the recommendation provided by Fiat Lux, I suggest clearing cache on your browser.  In addition, you can try to log into QuickBooks Online from another browser such as, Safari, Edge, Firefox, Google Chrome, Private or Incognito.  If you get the same result, follow these steps;

1. Log into QuickBooks Online

2. From the left menu open Transactions and then Banking

3. Click on the ▼ dropdown arrow located under the Update button

4. Your linked accounts should now appear

If you require additional assistance, please contact us so we can work with you directly and ensure you're able to accomplish your QuickBooks goals.

Schedule a Callback or start a Chat: click (?) Help in the upper right in QuickBooks Online > enter "Contact support" > hit Contact Us > provide a brief summary of your situation > hit Let's Talk > choose Get a callback or Start a Chat.
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If you have any other questions, please feel free to reach back out.  We'd love to help!