Recovering the deleted invoice in QuickBooks Self-Employed (QBSE) is currently unavailable. With this, I'd suggest re-creating it manually in your account. If you've sent the invoice before to your customers, you might want to ask them for a copy for your reference.
Once you have done creating the invoice, you have an option to copy, export as pdf, and send the transaction to your own email for your back up.
See the screenshot below for your reference:
Additionally, browse through these articles below. It contains different topics on how to manage customer's transactions, invoices, reports using a browser, and mobile apps.