I credited an invoice because I thought the customer had refused the product. I now know that the customer took the product and needs to pay. How do I reinstate the invoice so the customer can pay?

You can’t actually ‘reinstate’ the invoice since it’s been credited already. There are two ways to invoice the customer depending on how you did the credit.

Assuming a ‘sales return’ was done to credit the initial invoice: You now add another shipment to the SO (to replace the ‘refused product’ that was returned) Then allocate the goods and re-invoice

Or

Assuming you used the ‘Add New Invoice’ link and created a manual credit: You can use ‘Add New Invoice’ link again and create a debit Memo Invoice for the ‘refused’ product’ that was returned, to invoice the customer.