Please review the information at the following link for technical assistance instructions provided by Apple.
Apple Technical Assistance
1. Double-press your Home button to display all the current running apps at the bottom of the screen.
2. Tap and hold one of the application icons until they start to wiggle.
3. Tap on the red minus sign to close each application.
4. Press the Home button.
5. Reboot your iPhone/iPad/iPod Touch and launch the game again.
The game should now start.
If you're still having problems, please let us know exactly which of the following devices you're using...
iPhone 4, 4s
iPhone 5, 5s, 5c
iPhone 6, 6s
iPod Touch 1st Gen
iPod Touch 2nd Gen
iPod Touch 3rd Gen
iPod Touch 4th Gen
iPad (1st Generation)
iPad (3rd Generation)
iPad (4th Generation)
Also, please make sure to backup your phone and update your firmware (iOS). The most recent version is iOS 9.0.2.