An order will sometimes lose it line items due to corruption, whether it is a cloned order or a converted estimates. Usually it is due to corruption in the source order.

1. Rebuild the database
2. Edit the order
3. Add a line item, doesn't matter what the Product is (unless you want to recover the order then readd all the missing line items and modifiers exactly as it was before)
4. Save the order
5. Now you can void the order by clicking the Void button