To open a Closed Invoice, you will need to go into the customer's account screen either via the database:
Or via the Invoice Screen
There are three situations a close invoice can be in:
SITUATION 1: Invoice is closed but not yet received a payment against it
SITUATION 2: Invoice is closed but has a partial payment allocated
Fully paid invoices can only be found in the History Screen. If your invoice is fully paid, see SITUATION 3 next
When you exit the screen you will see that everything has been separated.
SITUATION 3: Invoice is closed and is fully paid