How do I restart an app?

See:

iOS: Troubleshooting applications purchased from the App Store

Restore from backup. See:

iOS: How to back up

Restore to factory settings/new iPod

From the home screen try double tapping the home button to bring up the multi-tasking bar. Hold your finger on the app you want to kill, Phonto, until it begins to shake. Then tap the red circle to force it to shut down. Press the home button and return to the home screen to try and launch the app again. If this dosn't help try killing all your running apps or rebooting your device.