Do you back-up to iTunes? If so the messages before should in the last backup made before you deleted them from your phone, the same applies to iCloud backups. You can try restoring from the last backup but be aware that you will lose anything saved to your phone since then.
There are third party apps and services that claim to be able to recover deleted message, if you Google (other search engine are available) for them you will find both free and paid for programs. How succesful they are I don't know so if you use them it is at your own risk.
