I'd be glad to help you out by sharing a possible scenarios I could think of.
Are you using a trial version of QuickBooks when creating invoices then subscribe for a different account? If that's the case, please make sure that you log in the username and password previously used.
That's the reason why the invoices no longer available since you've subscribed for a new QuickBooks account. While the trial versions have those invoices.
But if you're referring to the missing invoices in your QuickBooks account, let's check the Audit Log to verify if the invoices were deleted. Here's how: