Normally if a commodity line is closed from a manual modification of the procurement document, the fund can be added back to the same line in a new modification version.  Then you need to create a CEC document to reopen the award in order to continue processing payments.  The CEC creation job aid for this scenario can be found here.  

If a line has been closed due to a final reference on a payment document, you can restore the encumbrance with a CEC document without award modification.  The CEC creation job aid for this scenario can be found  here.