How to resend a missed out e-mail notification which was sent out for approval of Purchase
orders. 

Fix
To implement the solution perform the following steps:
1. Update the WF_NOTIFICATIONS table using the following script:

SQL>update wf_notifications set status ='OPEN', mail_status ='MAIL' where
notification_id=<NID>;
SQL> commit;

NOTE: HERE THE end_date COLUMN IS NOT UPDATED.

2. Then execute the following command:

SQL> execute wf_xml.EnqueueNotification(<NID>);
SQL>commit;

This should resend the notification mail.
Please note that only the notification mail will be resent. The workflow process will be not
rewind to the activity for which you have resent the notification.