How can I reinstate a old invoice that was deleted by mistake by other user?

Restoring a deleted invoice is unavailable, chew_william. 

But, I've got a way on how you can get the information of the deleted invoice and recreate it.

You can use the Audit log feature in QuickBooks Online. This helps track added, deleted, and modified transactions. 

Here are the steps:

1. Go to the Settings icon and select Audit Log.
2. On the Filter drop-down, choose the appropriate User, Date, and Events filter then, click Apply.
3. Locate the deleted transaction. To find deleted transactions, use the browser search Ctrl + F and type deleted.
4.  Select View under the HISTORY column.
5. Under the EVENT column, you'll see all the information.

You can check this article for additional info: Use the audit log.

If you need help customising your invoice, you can click this link for details and a guide on how: Customise invoices, estimates, and sales receipts in QBO.

I'll be right here to keep helping if there's anything else you'll need.