If IPN has been enabled but the IPN URL was entered incorrectly, you can correct the IPN URL and resend their IPN messages from the PayPal administration pages.
Orders that are up to 28 days old may be resent. Any orders older than that will not be available. Orders that are available for resend can be found in PayPal:
under the “My Account” tab, click “History”, then “IPN History”
If IPN was not enabled at the time the order was placed, then the order details can not be resent.