A closed purchase order can be opened by going into the details of the purchase order.
The purchase order needs to be closed [CL].
HOW IT WORKS
STEP 1: Go to the 'Closed' tab of the purchase order module
STEP 2: Open the details of the purchase order you want to close
STEP 3: Open the purchase order
Go to Actions --> Click 'Open Purchase Order'
Once you re-open a purchase order, it changes to the committed [CO] status. Please note, a committed purchase order is non editable, which means that it can be matched to invoices, however, to make changes it needs to be reset.