If it was deleted via archiving then you should be able to find the archive run in SARA, enter object MM_EKKO and click the administration button.
When did this happen? last week or some years ago? If recently then you may also find the job logs for archiving by clicking the job icon in SARA
About which history log are you talking? PO change logs get archived together with the PO, which means if you find document in the the change history table CDHDR and CDPOS and have nothing in EKKO table then you can be certain that someone did hard table updates (creating inconsistencies).
