An invoice will not automatically be closed unless a payment or a credit gets applied to it in QuickBooks Online (QBO), @robbie2. I'll help you resolve this and restore invoices to their normal statuses.
To begin, you'll want to access the invoice and check if there's a payment linked to the transaction. If there is, here's how you can remove it:
- Access your QuickBooks Online company.
- On the left navigational bar, go to the Customer & leads tab.
- Select Customers and then choose someone who you want to review the transaction list.
- In the Transaction List section locate the closed invoice and then click View/Edit. If you see 1 payment made on (specific date), click on it and then select the date that appears.
- Uncheck the box beside the invoice transaction where the payment was applied.
- One done, click the Save and close button.
For visual reference, see the image below.



You'll also want to ensure the Automatically apply credits are inactive in your Account and settings. This way, whenever you have available customer credits, they won't be associated with any open transactions they have in QBO. For more details, please refer to Step 1 in this article: Create and apply credit memos or delayed credits in QuickBooks Online.
On the other hand, you can customize invoices to personalize them and match the business you offer to customers in QBO. Refer to this article for more details: Customize invoices, estimates, and sales receipts in QuickBooks Online.
Aside from handling invoice transactions in QuickBooks, I can also assist you in reviewing customer transactions to ensure all recordings in your company file are on point. Please let me know in the comment section so I can respond promptly. Keep safe.