The Messages app would be the place to view your iMessages and SMS messages unless you are using a 3rd-party messaging app. Having said that, I'm going to go out on a limb and assume you're not able to read the message from the Notification Center or via the lockscreen. If so, go to Settings - Notifications - Messages and then select one of the choices next to "Show in Notification Center". Also, scroll down to the bottom and enable the toggle next to "Show Previews".