Once you’ve deleted transactions, there is no Undo after that. You can view the history from the Audit Log.
Click the “Gear” icon at the top right corner > Tools > Audit Log
Select Transactions and Show “Deleted/Voided Transactions”.
Locate deleted purchase order and click “View” under History.
That will show Audit History.
You can refer to this article for more information:
Screenshot for reference: