Why isn't an order number appearing on my recently created invoice?

I recently created an invoice to send to a customer to pay for a product and emailed this to him i have done this several times without issue . The customer informs me he has paid but i have no funds or postal details . i notice on previous drafts to other customers that an order number was created yet it would not appear to be the case with this order. yet i followed my usual method of creating an invoice and emailing to the customer . any help would be appreciated .