What happens if you try to add it in PO-A?  Try to just enter the header and no line items, then save it, then bring it back up and see if the lines are there.  If not, add the lines with the balance due so you can continue to receive against it.  If PO-A won't allow you to use the same PO number, use a different number (like 1) and enter the header.  Then save it and use Maintain Database on BKAPPO and change the PO number to what it should be.  Then bring it up in PO-A and see if the lines appear.

It is possible to delete a PO but they would have gotten an "are you sure?" type message so it is hard to do by accident.

I agree, that's probably the easiest way to fix it - I am not sure how it is happening.