I'm here to lend a hand to get those invoices back.
Once you delete an invoice in QuickBooks Online (QBO), the option to undo it is unavailable. To bring back the deleted invoice, you can use the Audit Log report and recreate them manually.
To do this, here's how:
Click the Gear icon at the top of your QBO account.
Select Audit log.
Set the Date to when you deleted the invoices.
Under Events, select Deleted/Voided transactions.
Look for the deleted invoices, then click on View.
Under the History column, get all the details that you need.
Once done, click the + New button and select Invoice. Then, enter all the details and click Save.