I can't find an invoiced purchase order.

When you completely turn a the entirerity of a purchase order into invoice(s):

1. Sage 50 turns the purchase order into invoice(s).
2. Sage 50 removes the purchase order.
3. Therefore you cannot find it anymore under the Purchase Order window.

If you want to be able to find Invoiced Purchase Orders by purchase order number you need to either:

Option 1: Use the same number for purchase invoices and purchase orders

1. On Sage main window, in the Vendors/Payable module, click Purchases Invoices or Purchases, Orders and Quotes icon.
2. In Purchases - Creating an Invoice window, click Purchase, Lookup Invoice.
3. Enter the right parameters then search for the Invoice (or Invoiced Purchase Order).

Option 2: Upgrade to Sage 50 Quantum Edition which includes the added lookup feature for invoiced purchase orders

1. In Enhanced View, on Sage main window, in the Vendors/Payable module, click Purchase Orders icon.
2. In Classic View, on Sage main window, in the Vendors/Payable module, click Purchases, Orders and Quotes icon, then choose Transaction type Orders.
3. In Purchases - Creating an Invoice window,on the right side of the Order No.: field, click on the Magnifying Glass icon.
4.  In the Select Order or Quote window, put a check mark in the box for Invoiced Orders and remove the check mark for Uninvoiced Orders and Quotes.
5. Look up for the Purchase Order with the status Invoiced.