Does anyone know how to reconnect a banking account that was disconnected?

You are in the right place for help, @davekingmb.

I can walk you through the steps of reconnecting your American Express account in QuickBooks.

You'll just have to navigate from the banking page and reconnect the account from there. Let me give you the detailed steps:

1.  Open the Banking menu.
2. Choose Connect Account or Add Account.
3.  Search for your bank and hit Continue once you've located it.
4. Enter the credentials you use to log in to the bank's website.
5. Choose the accounts you'd like to connect and assign an appropriate account type.
6. Select the date range and hit Connect.

For more details about managing bank accounts and fix errors downloading transactions in QBO, feel free to check out these guides: 

• Connect bank and credit card accounts to QuickBooks Online.
• What to do if you get a bank error or can't download transactions in QuickBooks Online

Please let me know if I can be of additional help with QuickBooks and banking concerns. I'm ready to back you and your business. Take care always.