On iOS (iPhone/iPad), try closing the app completely from the home screen of your device. To do this double-click the home button to bring up the active apps, then simply 'flick up/away' any of the apps on the screen to close them completely. Click the home button again to leave this mode. Then try starting the app up again.
If that doesn’t work, then you could try a reset of your device. To do this press and hold both the sleep and home buttons for about 10 to 15 seconds (ignore the red slider), after which the Apple logo should appear. Don't worry, you won’t lose any content.