I just encountered the same problem.  This worked for me, so hopefully it works for others too.   I opened the PO in question, selected the "Marked As Closed" button, and selected "Save and Close".   I then looked up the same PO, selected the "Mark As Open" button, and selected "Save and Close".  I then tried doing a receipt against the PO, and it appeared the list of available POs for receiving,and I was able to process the receipt for the remaining items that had arrived.