Inadvertently deleted an unpaid invoice from back in January 2020. Any way of recovering this ?

Good day, tony93.

For the time being, we're unable to restore a deleted transaction. However, you can retrieve the information in the Audit Log report. 

Here's how:

1. Go to the Gear > Audit Log.
2. Click the Filter drop-down and select the User, Date, and put a check mark on the Transactions box under Events.
3. Choose Deleted/Voided Transactions from the Show drop-down.
4. Hit Apply.
5. Search for the deleted invoice and click the View link to see the history.

Please see this sample screenshot for a visual guide: 

Once you have the details, you can proceed in creating the invoice. Feel free to check out this article that includes a video tutorial: QuickBooks Online - Invoicing and quoting tutorials.

I'm here if you have more questions about invoices or other concerns with QuickBooks Online. Take care and stay safe.