Doubletap the home button and that will bring up the multi-tasking bar. Hold down on the Messages app until you see the minus sign. Tap on the minus sign to close the app. Tap the home button again. Try to open the Messages app. If you still have trouble, do a reset on the phone. Hold the sleep/wake and home buttons together until you see the Apple logo and then release. Once the phone restarts, try to open Messages again.
Close All Open Apps... Perform a Reset... Try again...
Reset ( No Data will be Lost )
Press and hold the Sleep/Wake button and the Home button at the same time for at least ten seconds, until the Apple logo appears. Release the Buttons.