If you'd like to restore messages that have been deleted from the Messages app, one option would be to have the original sender copy and send you the message once more. Alternatively, you can restore your iPhone from a backup if you have one that was created at a point that the message was still on your device. Keep in mind that any data created or added to the phone that isn't stored in iCloud (or otherwise synced to another service) will be lost unless you archive the backup that you'd like to restore from, then create a current backup.