I'd like to ask if you accidentally deleted it or you just can't locate it. Please note that we're unable to recover the invoice if it was deleted. The best option is to use the Audit Log so we can collect its data, then recreate it.
Select the deleted invoice to see the details. Copy it and create a new invoice.
If just can't locate it, there are many ways on how we can pull it up. First, you can use the Search bar(magnifying glass icon) to search it using transaction number, amount, or date. Another way is by going to the client's transactions list.
From the Sales tab, select Customers.
Click the customer's name.
On the Transaction List section, search for the said invoice.
We can also pull up the invoice list report. Go to the Reports tab, search and select the Invoice List, then you can customize it.
I have some articles here for more details about invoices: