Not sure if you are using AME.
1. You can re-assign the notification to an active employee through re-assign option from workflow administrator responsibility> Status Monitor
2. If you are in R12, use the Management> Forward Document form and re-assign the notification
3. If the either of the above doesn't work then kindly Reset the requisition to incomplete status and then resubmit it for approval, make sure if the new employee is replaced with the terminated one.
How To Reset a Document To Incomplete/Requires Reapproval For Isolated Cases Doc ID 390023.1