Thanks for reaching out about this. I know it can be concerning to have information missing when you've spent time entering it into your books. I want to make sure you're able to track down what happened with your client list and invoices so you can quickly get back on track.
The Audit log in QuickBooks Online is a powerful tool that enables you to monitor what's going on in your account. It's a great way to keep track of your steps and know who did what in your books. I strongly encourage you to have a look over there to see if there's any record related to your missing information. Simply go to the Gear icon then Audit log to access this feature. Here's an article for more details on that: Use the audit log in QuickBooks Online