If your PO is soft closed means closure status = 'closed' then you can reopen the PO.
If your PO is hard closed means closure status = 'finally closed' then you cannot reopen the PO.
The Navigation path is as follows to open the PO.
1. Open PO Summary Form
2. Query with Existing PO.
3. On PO Header Screen, Select Menu > Tools > control > Actions = Open > ok