How do I assign an invoice number to an invoice from a closed year?

Go to Reports & Forms, Forms
Select Invoices and Packing Slips or Credit Memos in the menu which appears.
Select the desired invoice form in the Forms box on the left hand side of the Select a Report or Form window.
Click on the Preview and Print button located towards the right hand side of this page.
Select Unprinted invoices under the Invoices to print/e-mail option.
Click Refresh List in the upper right corner of this window.
Place a check next to the desired invoices in the list and then click on the Print/E-mail button at the bottom.
This will result in Sage 50 assigning invoices numbers to the invoices which will make them available to see in the Receipts window.