If you deleted the message yourself, check the Trash folder on your eBay Messages page:
https://mesg.ebay.com/mesgweb/ViewMessages/5
You may need to switch back to the "Classic" message view if you are using the "New" message view to see that link; I do not thing the "New" view supports folders yet.
If eBay removed the message due to age, I do not think there is a way to get it back. If you have your eBay messages forwarded to an email address, check your email archives to see if you have a copy saved there.
