How do I recover a lost invoice, spent ages making and it hasn't saved?

No chance......

Slow 🐌 Books does not have an auto save function.......and unless you save regularly....you will always loose data

Hello there, shonadoran@yahoo.

QBO doesn't automatically save transactions without clicking the Save or Save and close button. If not saved, we're unable to recover them.

With that, ShopAdmin is right. You'll have to need to manually save the invoice you've created in the system.

You'll want to sign in to your QuickBooks Online account to send your valuable feedback to our Product Development team

1. Go to the Gear icon.
2. Select Feedback under Profile.
3. Enter your suggestion. Then, click Next.

I've added this link that you can utilize about managing invoices: Customise invoices, estimates, and sales receipts in QuickBooks Online.

We always want to hear your suggestions as we continue developing the product as best for your business. Take care.