If you have access, look at "View Approval through Workflow" under the Inquire menu - it'll give you more detail on what's happened and what actions have been tried.
Q 1 - Are you using Encumbrance ? If yes, this is a common problem. We face it quite often.
Q 2 - Did you check if the user received the rejection notification.
Login as "Sysadmin" user, navigate to the function status monitor
Search for your requisition using PO number in the User Key field
Look for the process which as failed.
A typical solution would be to run a standard Oracle script to bring the PO back to Incomplete status, re-submit and on approval close it.
There is a generic datafix script shipped as a patch. This script will help in resetting the document to Incomplete / Requires Reapproval status. There after you can adjust the PO to have correct quantities and re-approve it.
The datafix script is $PO_TOP/sql/poxrespo.sql. Please read the documentation and try out in test instance first.