If you don't have a backup, there is no way. Sorry.
What you could do for the future is install SMS Backup+ from the Play Store. This backs up SMS to your Gmail account, and can be set to do it automatically when messages are sent or received (you can specify a delay so that it backs up a conversation in one go rather than message by message). Or there are many other apps which can back then up to your SD card on a regular schedule. But none of this will help you now, I'm afraid.