Why does a PO disappear? I received a partial from a vendor and now the PO no longer appears on the Open PO report nor can I receive any of the other items.

I received items for a partial shipment from a vendor. Now, that PO no longer shows up in my open POs report nor on the list of POs from that vendor when I try to receive more items. The PO still shows as an open PO when I manually open it, but I cannot receive items against it.

HELP!