So, have you accidentally deleted them from your iPhone? I did the same thing a couple weeks a go. Here is the solution I found:
If you have back up your text messages, you can restore from the backup.
Look at this tutorial from Apple: http://support.apple.com/kb/ht1766
If you have no backup for your data on iPhone, iPhone data recovery software is your last shot! I used Tenorshare iTunes Data Recovery and it worked well. Give it a shot! Get it from http://goo.gl/KWeoQ