I can see how inconvenient it is for you, @mlevy.
Please know this not what I want you to experience with QuickBooks.
Let me route you to our dedicated team that best handles this kind of situation. I recommend reaching out to our Merchant Support team. They have the tools to securely check your account and can advise you on the best action to take in dealing with your customer dispute.
Once everything is settled, you can refer to this article for the detailed steps on how to accept payment for the online invoices you've sent: Take and process payments in QuickBooks Online.
Let me know if you have other follow-up questions about managing your invoices in QuickBooks. I’ll be around ready to answer them for you. Take care and always stay safe.